Melt 4 tablespoons of butter in a medium saucepan over medium heat. Whisk in ¼ cup of all-purpose flour for 2 minutes until golden. Gradually add 2 cups of milk, whisking for about 5-7 minutes until thickened. Stir in nutmeg, and season with salt and pepper.

In a large skillet, heat 2 tablespoons of olive oil. Sauté one chopped onion and 3 cloves of minced garlic for 2-3 minutes. Add diced red and yellow peppers, cooking for another 3-5 minutes. Stir in zucchini and well-drained frozen spinach, cooking until heated through. Allow to cool slightly.

In a large bowl, combine 15 ounces of ricotta cheese, ¾ cup of grated Parmesan cheese, and 1 beaten egg. Mix thoroughly, then gently fold in the cooled vegetable filling.

Bring a large pot of salted water to a boil. Cook lasagna noodles according to package instructions (about 8-10 minutes for regular). Drain and rinse with cold water.

Preheat your oven to 375°F (190°C). Grease a 9x13-inch baking dish. Spread a thin layer of white sauce, layer with noodles, vegetable filling, and mozzarella. Repeat layering until all ingredients are used, finishing with a layer of sauce and mozzarella.

Cover with aluminum foil and bake for 25 minutes. Remove the foil and bake for another 20-25 minutes until cheese is bubbling and golden.

Let the lasagna rest for 10-15 minutes before slicing and serving.