In a medium saucepan, melt 4 tablespoons of butter, whisk in 4 tablespoons of all-purpose flour, and cook until golden.

Gradually pour in 2 cups of milk, whisking continuously until smooth, then simmer until thickened.

Stir in a pinch of nutmeg, salt, and pepper, then set aside to cool.

In a large skillet, heat 2 tablespoons of olive oil, sauté 1 diced onion until softened.

Add 2 minced garlic cloves, 1 diced bell pepper, and 1 diced zucchini; cook until tender-crisp.

Stir in 2 cups of drained frozen spinach, season with salt and pepper, and let cool.

In a mixing bowl, combine 2 cups of ricotta cheese, ½ cup of grated Parmesan cheese, and 1 beaten egg.

Fold in the cooled vegetable mixture gently.

Cook lasagna noodles in boiling salted water according to package instructions, then drain and rinse.

Preheat the oven to 375°F (190°C) for assembly.

In a baking dish, layer white sauce, 3 noodles, half cheese filling, and mozzarella.

Repeat layers, finishing with noodles and remaining white sauce topped with mozzarella.

Cover with foil and bake for 25 minutes; remove foil for another 20-25 minutes until golden.

Allow to rest for 10-15 minutes before slicing.