In a medium saucepan, melt ½ cup of butter over medium heat until bubbly. Whisk in ½ cup of flour and cook for 1-2 minutes until smooth. Gradually add in 4 cups of milk, stirring constantly until the mixture thickens, about 5-7 minutes. Season with ¼ teaspoon nutmeg, ½ teaspoon salt, and ¼ teaspoon black pepper. Remove from heat and set aside to cool slightly.

Heat 1 tablespoon of olive oil in a large skillet over medium heat. Add 1 chopped onion and sauté for 3-4 minutes until softened. Stir in 2 minced garlic cloves, cooking for another 1 minute. Next, add the chopped red and yellow bell peppers and 1 chopped zucchini, cooking for about 5-7 minutes until tender. Fold in 10 ounces of thawed and drained frozen spinach, seasoning with salt and pepper. Allow the mixture to cool.

In a mixing bowl, combine 15 ounces of ricotta cheese, ½ cup of grated Parmesan cheese, and 1 lightly beaten egg. Gently fold in the cooled vegetable mixture, ensuring it's well combined. This will be the flavorful filling for your Loaded Veggie White Lasagna. Set aside while you proceed to prepare the lasagna noodles.

Bring a large pot of salted water to a boil. Add 9 lasagna noodles and cook according to package instructions, usually about 8-10 minutes, until al dente. Drain the noodles and rinse them under cold water to prevent sticking. Lay them flat on a clean kitchen towel while you get ready to assemble the layers.

In a 9x13 inch baking dish, spread a thin layer of the white sauce evenly on the bottom. Layer three lasagna noodles over the sauce, followed by half of the vegetable filling and a sprinkle of shredded mozzarella cheese. Repeat the layering process, finishing with a final layer of noodles, topping with remaining white sauce and a generous amount of mozzarella cheese.

Preheat your oven to 375°F (190°C). Cover the assembled lasagna with aluminum foil and bake for 25 minutes. Remove the foil and continue to bake for an additional 20-25 minutes, or until the cheese is bubbly and golden brown. Keep an eye on it to ensure the cheese doesn’t over-brown.

After baking, remove the lasagna from the oven and let it rest for 10-15 minutes. This cooling period allows the layers to set, making it easier to slice and serve. The Loaded Veggie White Lasagna will be perfectly creamy and deliciously inviting when you finally dig in!