Prepare fettuccine according to package directions, reserving 1 cup cooking water. Set aside.
Into the same pot used for cooking pasta, add butter. Set heat to medium and allow butter to melt.
Add spinach leaves and cook 2-3 minutes, until beginning to wilt.
Add garlic, salt, and pepper. Continue cooking another 1-2 minutes, until spinach is wilted.
Add cooked pasta, parmesan cheese, and 1/2 cup of the reserved pasta water. Gently toss to coat, adding more reserved water as needed.
Adjust seasoning to taste.
Divide paste between serving bowls. Sprinkle with additional Parmesan cheese and red pepper flakes. Serve hot.