Preheat your oven to 375°F (190°C).

Melt 4 tablespoons of unsalted butter in a medium saucepan over medium heat and whisk in 1/4 cup of all-purpose flour. Cook for 2 minutes until it bubbles but does not brown.

Gradually add 2 cups of whole milk while continuously whisking for 5-7 minutes until the sauce thickens.

Season the béchamel sauce with salt and pepper, then set aside.

Cook the lasagna noodles according to package instructions until al dente, about 8-10 minutes. Drain and rinse under cold water.

In a greased baking dish, layer with béchamel sauce, noodles, cheese blend, spinach, and optional mushrooms or chicken. Repeat layers and finish with béchamel and cheese on top.

Bake for 45 minutes in the preheated oven until golden and bubbly.

Let the lasagna rest for 10-15 minutes before slicing to serve.