In a medium saucepan over medium heat, melt 4 tablespoons of butter. Whisk in 1/3 cup of all-purpose flour, stirring continuously for about 2 minutes until bubbly. Slowly add 3 cups of milk while whisking to avoid lumps, and cook until thickened, roughly 5-7 minutes. Sprinkle in a pinch of nutmeg, salt, and black pepper for flavor; set aside.

In a large skillet, heat 2 tablespoons of olive oil over medium heat. Add 1 diced onion and sauté for about 3-4 minutes until soft. Stir in 3 minced garlic cloves, followed by 1 cup each of chopped red and yellow bell peppers and diced zucchini; cook for an additional 5 minutes until tender. Add 2 cups of drained frozen spinach, season with salt and pepper, and allow the mixture to cool slightly.

In a mixing bowl, combine 15 ounces of ricotta cheese, 1/2 cup of grated Parmesan, and 1 beaten egg. Fold in the cooled vegetable mixture, ensuring everything is well incorporated.

Bring a large pot of salted water to a boil and cook 9-12 lasagna noodles according to package instructions until al dente. Drain the noodles and gently rinse them under cold water to prevent sticking.

In a 9x13 inch baking dish, spread a thin layer of the white sauce on the bottom. Place three lasagna noodles on top, followed by a layer of the vegetable filling. Top with a generous sprinkle of mozzarella cheese, then repeat the layers: white sauce, noodles, filling, and mozzarella. Finish with a final layer of noodles topped with remaining white sauce and mozzarella cheese.

Preheat your oven to 375°F (190°C). Cover the assembled Loaded Veggie White Lasagna with aluminum foil to prevent burning, and bake for 25 minutes. Remove the foil and continue baking for another 20-25 minutes, until the cheese is bubbly and golden brown. Allow the lasagna to cool for 10-15 minutes before slicing and serving.