Melt 4 tablespoons of unsalted butter over medium heat in a saucepan. Whisk in 1/4 cup of all-purpose flour until smooth, cooking for about 1 minute. Gradually add 3 cups of whole milk, simmering until thickened, then stir in 1/4 teaspoon nutmeg, salt, and black pepper.

Heat 2 tablespoons of olive oil in a large skillet over medium heat. Add 1 chopped onion, sautéing until soft—about 4 minutes. Stir in 2 minced garlic cloves, 1 red bell pepper, 1 yellow bell pepper, and 1 diced zucchini, cooking until tender, approximately 5-7 minutes. Mix in 1 cup of drained frozen spinach, seasoning with salt and pepper. Let cool slightly.

In a large bowl, mix 1 cup of ricotta cheese, 1/2 cup of grated Parmesan cheese, and 1 egg until well combined. Gently fold in your cooled vegetable mixture until evenly distributed.

Prepare 12 lasagna noodles according to package directions. Drain and rinse with cold water to stop the cooking process.

Preheat the oven to 375°F (190°C). Spread a layer of white sauce on the bottom of a baking dish. Layer 4 noodles, half of the vegetable filling, and 1 cup of shredded mozzarella. Repeat this twice, finishing with the last 4 noodles topped with the remaining white sauce and 1 cup of mozzarella.

Cover the baking dish with foil and bake for 25 minutes. Then, remove the foil and bake for another 20-25 minutes until the cheese is bubbly and golden brown.

Remove the lasagna from the oven and allow it to rest for 10-15 minutes. This helps set the layers, making it easier to slice. Serve and enjoy!