Prepare the Béchamel by melting 4 tablespoons of butter in a saucepan and whisking in 1/4 cup of flour until golden. Gradually add 2 cups of milk, whisking until thickened. Season with salt, pepper, and nutmeg.

Sauté the Fillings in a skillet with 1 tablespoon of olive oil, then add the mushrooms and cook until tender. Stir in the spinach until wilted, then mix with 1 cup of ricotta and half the cheese blend.

Assemble the Layers by preheating the oven to 375°F. Spread some béchamel in a baking dish, layer with noodles, filling, and more béchamel until complete, ending with béchamel on top.

Bake the Lasagna covered for 25 minutes. Uncover and bake an additional 15 minutes until bubbling and golden.

Let It Rest for 15 minutes after baking to set before slicing.