Preheat the oven to 350°F. Line a baking sheet with parchment paper. Spray an 8x8-inch baking dish with non-stick cooking spray.
Spread bread cubes on a baking sheet in a single layer. Toast for 10 minutes, until dry and lightly golden. Set aside.
Set a large skillet over medium heat and add 3 tablespoons of butter. When the butter is melted, add the onion and celery. Cook for 3 minutes, stirring frequently, until softened.
Stir in sage, parsley, and thyme and cook 1 minute. Remove from heat.
In a large bowl, gently stir together the prepared mashed potatoes, toasted bread cubes, and sautéed vegetables. Add the 2 tablespoons of melted butter. Drizzle in broth a little at a time, stirring gently, until mixture is moist but not soggy. Adjust seasoning to taste with salt and black pepper.
Transfer to the prepared baking dish. Bake at 350°F, uncovered, for 45–55 minutes, until the top is crisp and golden.