In a medium saucepan, melt 4 tablespoons of unsalted butter over medium heat. Whisk in ¼ cup of all-purpose flour until smooth, and cook for about 1-2 minutes until bubbly. Gradually add 3 cups of milk, whisking continuously until thickened, about 5-7 minutes. Stir in a pinch of nutmeg, salt, and black pepper to taste, and set aside to cool slightly.

Heat 2 tablespoons of olive oil in a large skillet over medium heat. Sauté 1 chopped onion for 3-4 minutes until translucent, then add 2 minced garlic cloves, 1 diced red bell pepper, and 1 diced yellow bell pepper. Cook for 5-6 minutes until the vegetables are tender. Stir in 1 cup of drained frozen spinach, season with salt and pepper, and cook for another 2 minutes. Let the mixture cool slightly.

In a mixing bowl, combine 15 ounces of ricotta cheese, ½ cup of grated Parmesan cheese, and 1 beaten egg. Mix well until creamy, then gently fold in the cooled vegetable filling from Step 2 until fully incorporated.

Prepare lasagna noodles according to package instructions, making sure to choose no-boil noodles for an easier assembly. Once cooked, drain the noodles and rinse them under cold water to stop the cooking process. Lay them flat on a clean kitchen towel.

In a 9x13-inch baking dish, pour a thin layer of the white sauce on the bottom. Lay down a layer of lasagna noodles, followed by half of the vegetable cheese filling, and top with 1 cup of shredded mozzarella cheese. Repeat the layering process until all ingredients are used.

Cover the assembled lasagna with aluminum foil, ensuring it doesn't touch the cheese. Bake in a preheated oven at 375°F for 25 minutes. Remove the foil and bake for an additional 20-25 minutes, or until bubbly and golden brown.

Once out of the oven, let the lasagna rest for 10-15 minutes before serving.