Preheat your oven to 375°F (190°C). Prepare your 9x13-inch baking dish.

In a medium saucepan, melt 4 tablespoons of butter over medium heat. Whisk in 1/4 cup of flour until combined, then gradually add 3 cups of whole milk while stirring until thickened (about 5-7 minutes).

In a large skillet, heat olive oil and cook 1 pound of sausage (or mushrooms) until browned, about 7-10 minutes. Drain excess fat.

Layer the ingredients starting with béchamel sauce, followed by no-boil noodles, ricotta, mozzarella, spinach, and sausage. Repeat layers.

Pour any remaining béchamel over the top, then sprinkle with shredded mozzarella and Parmesan cheese.

Cover the baking dish tightly with foil and bake for 25 minutes.

After 25 minutes, remove the foil and continue baking for another 15-20 minutes until golden-brown and bubbly.

Let the lasagna rest for 10 minutes before slicing and serving.