In a medium saucepan over medium heat, melt 4 tablespoons of butter until bubbly. Gradually whisk in ¼ cup of all-purpose flour, stirring constantly for about 1 minute until lightly golden. Slowly add 3 cups of milk, continuing to whisk until the mixture thickens, about 5–7 minutes. Season with a pinch of nutmeg, salt, and black pepper. Set this delicious Loaded Veggie White Lasagna sauce aside to cool.

In a large skillet, heat 2 tablespoons of olive oil over medium heat. Add one chopped onion and sauté for about 3 minutes until translucent. Stir in 2 minced cloves of garlic, followed by diced red and yellow bell peppers, and 1 sliced zucchini. Cook for another 5 minutes, or until vegetables are tender-crisp. Add 2 cups of drained frozen spinach, cooking just until heated through; season this colorful mixture with salt and pepper.

In a mixing bowl, combine 1 cup of ricotta cheese, ½ cup of grated Parmesan cheese, and one large egg. Blend together until smooth and creamy. Gently fold in the cooled vegetable filling, making sure to evenly distribute the veggies.

Bring a large pot of salted water to a boil, and cook 9-12 lasagna noodles according to package instructions, usually about 8–10 minutes for traditional noodles. Drain the noodles thoroughly, rinsing them under cold water to stop the cooking process and prevent sticking.

Preheat your oven to 375°F (190°C). In a greased 9x13 inch baking dish, spread a layer of the prepared white sauce on the bottom. Place a single layer of lasagna noodles over the sauce, followed by half of the cheese and vegetable mixture. Then, layer with more sauce and noodles. Repeat the layers, finishing with noodles topped generously with the remaining white sauce and 1 cup of shredded mozzarella cheese.

Cover the baking dish with aluminum foil and bake in the preheated oven for 25 minutes. Remove the foil and bake for an additional 20-25 minutes until the cheese is bubbly and golden brown. Let it sit for about 10 minutes before slicing to allow the layers to set.