In a saucepan over medium heat, melt 6 tablespoons of butter. Whisk in 6 tablespoons of flour to create a roux, and cook for about 2 minutes until golden.

Gradually pour in 4 cups of whole milk, stirring continuously for 8-10 minutes until the sauce thickens and coats the back of a spoon. Season with 1 teaspoon of salt and a pinch of nutmeg.

Preheat your oven to 375°F (190°C).

In a greased 9x13-inch baking dish, spread a thin layer of béchamel sauce at the bottom. Lay down a layer of no-boil lasagna sheets so they slightly overlap.

Add a generous layer of the cheese blend, which includes mozzarella, ricotta, and parmesan, along with any optional ingredients like spinach or mushrooms, if desired.

Continue layering by adding more béchamel sauce, lasagna sheets, and cheese mixture, aiming for at least three layers. Ensure the top layer consists of béchamel sauce and a heavy sprinkle of cheese blend.

Cover the baking dish with aluminum foil, ensuring it doesn't touch the cheese. Bake for 25 minutes, then remove the foil and bake for an additional 20 minutes until golden brown and bubbling.

Let it rest for 10-15 minutes before serving to allow the layers to set properly.