In a medium saucepan, melt 4 tablespoons of butter over medium heat. Whisk in ¼ cup of all-purpose flour, stirring constantly for about 2 minutes. Gradually add 2 cups of milk, whisking until smooth and thickened (about 5 minutes). Season with a pinch of nutmeg and black pepper, then remove from heat.

Heat 2 tablespoons of olive oil in a large skillet over medium heat. Sauté 1 chopped onion and 2 minced garlic cloves until softened, about 3-4 minutes. Add 1 diced red bell pepper, 1 diced yellow bell pepper, and 1 sliced zucchini. Cook for another 5-7 minutes until tender. Stir in 1 cup of frozen spinach, season with salt and pepper, then remove from heat.

In a large mixing bowl, combine 15 ounces of ricotta cheese, ½ cup of grated Parmesan cheese, and 1 beaten egg. Fold in the cooled vegetable mixture until evenly combined.

Bring a pot of salted water to a boil. Cook 9 lasagna noodles according to package instructions (about 8-10 minutes) until al dente. Drain and lay flat on a clean towel to cool.

Preheat your oven to 375°F (190°C). In a greased 9x13 inch baking dish, layer components: spread a thin layer of white sauce, then place 3 noodles on top. Add half of the veggie cheese filling, then 1½ cups of shredded mozzarella. Repeat layering, finishing with white sauce and mozzarella on top.

Cover with aluminum foil and bake for 25 minutes. Remove foil and bake for an additional 20-25 minutes until cheese is golden and edges are sizzling.

Allow the lasagna to rest for 10-15 minutes. This cooling period helps the layers set and makes it easier to slice.