Caramelized Onion Spaghetti

Caramelized Onion Spaghetti is a vegetarian pasta made with slow-cooked Vidalia onions, briny capers, white wine, and Parmesan tossed with spaghetti and finished with fresh parsley. Serve with garlic bread or Roasted Leek Greens Salad.

Let’s Talk About These Onions

This recipe starts with what looks like an alarming amount of raw onion, 6 full cups to be exact. By the time you’re done, those onions will have collapsed into about 1½ cups of deeply golden, sweet, and intensely flavored caramelized onion. That’s your sauce. That’s where all the flavor lives.

The process takes 35 to 40 minutes over the lowest heat your stove allows, and it cannot be rushed. Turn up the heat, and you’ll brown the onions rather than caramelize them – similar in appearance but completely different in flavor. Low and slow is the only way.

Vidalia onions are ideal here. Their naturally high sugar content caramelizes beautifully, giving the sauce its distinctive sweetness. Yellow onions are a solid substitute when Vidalias aren’t in season.

What Makes This Pasta Special

I like this dish because the caramelized onions are the heart of the recipe. What starts as a skillet piled high with raw onions slowly cooks down, becoming intensely flavorful. There’s no cream or complicated technique, so this dish is easy to make, but it does require time and a little patience while the onions caramelize. If you’ve tried my French Onion Pasta, you’ll find this is a lighter dish with a similar method.

Once the onions are done, the rest of the recipe moves quickly. Garlic, capers, and red pepper flakes go into the pan, followed by a little butter and a splash of dry white wine to deglaze everything and tighten the sauce. The capers also deserve a mention here. Finely chopped and stirred into the sweet caramelized onions, their briny, salty bite is the perfect counterpoint to all that sweetness. Don’t leave them out.

The pasta finishes in the skillet, tossed vigorously with reserved pasta water and Parmesan until the sauce is glossy and coats every strand evenly. This is the technique that separates a well-made pasta from a great one — the starchy pasta water emulsifies with the Parmesan and onion sauce, making it cohesive and silky rather than just a pile of noodles with sauce sitting on top.

Ingredients for Caramelized Onion Spaghetti

Here’s what you will need. Exact amounts are listed in the recipe card below.

• Olive Oil — The base for cooking the onions low and slow.
• Vidalia Onions — Sweet and perfect for caramelizing. Yellow onions work well, too.
• Kosher Salt and Ground Black Pepper
• Fresh Garlic — Added after caramelization for a deep, rounded flavor without bitterness.
• Capers — Finely chop them so they distribute evenly through the sauce. Their brine is the secret balance to the sweet onions.
• Red Pepper Flakes — Optional for adding a gentle warmth.
• Unsalted butter — Stirred in for richness and a silky finish.
• Dry white wine — Pinot Grigio, Sauvignon Blanc, or unoaked Chardonnay all work beautifully. If you wouldn’t drink it, don’t cook with it, and avoid anything labeled “cooking wine.”
• Spaghetti — The long strands are ideal for catching the sauce.
• Parmesan — Added in two stages: into the sauce and again on top. Freshly grated melts more smoothly than pre-grated.
• Fresh parsley — A bright, fresh finish that lightens the richness of the dish.

To Make it Vegan: Use non-dairy butter and Parmesan cheese.

Pro Tip: A mandoline makes quick work of slicing onions and produces the thin, uniform slices that caramelize most evenly.

How to Make Caramelized Onion Spaghetti

STEP ONE: Caramelize the Onions. Set a wide skillet over low heat and add the olive oil. When the oil is hot, add the onions, salt, and black pepper. Cook, stirring occasionally, for 35 to 40 minutes until deeply golden. If the onions start to stick, add a small splash of water and use your spoon to scrape up any browned bits from the bottom of the pan. Do not rush this step.

STEP TWO: Build the Sauce. Add the garlic, capers, and optional red pepper flakes to the caramelized onions. Cook for 1 minute, just until fragrant. Add the butter and stir to combine.

STEP THREE: Deglaze. Pour in the white wine and stir, scraping up any browned bits from the bottom of the pan. Simmer for about 1 minute, until the sauce tightens slightly.

STEP FOUR: Cook the Pasta. While the onions caramelize, bring a large pot of generously salted water to a boil and cook the spaghetti until just al dente. Before draining, reserve 1 cup of pasta cooking water.

STEP FIVE: Finish in the Skillet. Transfer the pasta directly into the skillet over medium-low heat. Add ¼ cup of reserved pasta water and toss vigorously. Add half the Parmesan and toss again, adding more pasta water a splash at a time until the sauce is glossy and coats every strand.

STEP SIX: Finish and Serve. Adjust seasoning to taste, then top with the remaining Parmesan and fresh parsley. Serve immediately.

Storage and Reheating

This dish is best enjoyed fresh. If you have leftovers, store them in an airtight container in the refrigerator. Reheat on the stovetop over low heat with a splash of water to loosen the sauce.

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Caramelized Onion Spaghetti Recipe

A simple, elegant vegetarian pasta made with deeply golden Vidalia onions slow-cooked until jammy and sweet, then tossed with spaghetti, briny capers, white wine, and Parmesan.

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Course: Main Course

Cuisine: American, Italian

Keyword: caramelized onion spaghetti, sweet onion pasta, vegetarian, vidalia onion pasta

Prep Time: 10 minutes minutes

Cook Time: 50 minutes minutes

Total Time: 1 hour hour

Servings: 4

Calories: 687kcal

Author: Holly Gray

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Ingredients

• 3 tablespoons olive oil
• 6 cups Vidalia onions thinly sliced
• 3/4 teaspoon kosher salt divided
• 1/2 teaspoon ground black pepper
• 1 teaspoon garlic minced
• 1 1/2 tablespoons capers finely chopped
• 1/4 teaspoon red pepper flakes optional
• 1 tablespoon butter
• 1/4 cup dry white wine
• 16- ounces spaghetti by weight
• 3/4 cup grated Parmesan divided
• 2 tablespoons parsley chopped

Instructions

• Set a wide skillet over low heat and add the olive oil. When the oil is hot, add the onions, salt, and black pepper. Cook, stirring occasionally, for 35 to 40 minutes until deeply golden. If the onions start to stick, add a small splash of water and use your spoon to scrape up any browned bits from the bottom of the pan. Do not rush this step.
• Add the garlic, capers, and optional red pepper flakes to the caramelized onions. Cook for 1 minute, just until fragrant. Add the butter and stir to combine.
• Pour in the white wine and stir, scraping up any browned bits from the bottom of the pan. Simmer for about 1 minute, until the sauce tightens slightly.
• Cook the Pasta. While the onions caramelize, bring a large pot of generously salted water to a boil and cook the spaghetti until just al dente. Before draining, reserve 1 cup of pasta cooking water.
• Finish in the Skillet. Transfer the pasta directly into the skillet over medium-low heat. Add ¼ cup of reserved pasta water and toss vigorously. Add half the Parmesan and toss again, adding more pasta water a splash at a time until the sauce is glossy and coats every strand.
• Finish and Serve. Adjust seasoning to taste, then top with the remaining Parmesan and fresh parsley. Serve immediately.

Nutrition

Calories: 687kcal | Carbohydrates: 106g | Protein: 23g | Fat: 18g | Saturated Fat: 5g | Polyunsaturated Fat: 2g | Monounsaturated Fat: 9g | Cholesterol: 17mg | Sodium: 881mg | Potassium: 610mg | Fiber: 6g | Sugar: 15g | Vitamin A: 382IU | Vitamin C: 15mg | Calcium: 250mg | Iron: 3mg

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