Preheat the oven to 375°F.

Cook sausage in a large pan over medium-high heat until no longer pink. Stir often to crumble the sausage.

Add chopped onion, garlic, and dried basil and stir to combine. Cook for 5 minutes.

Add sliced mushrooms and cook for another 5 minutes.

Stir in diced tomatoes, tomato paste, and pepper. Bring to a boil, then reduce heat and simmer for 15 minutes.

Add spinach, stir, and remove from heat.

In a small saucepan, melt butter over medium heat. Whisk in flour and cook for 1 minute.

Gradually whisk in milk and cook until thickened. Stir in 1 cup mozzarella and a pinch of salt.

Cook spaghetti until al dente, then drain and combine with meat sauce.

Spread the mixture in a 9x13 inch casserole dish. Pour cheese sauce on top, then sprinkle remaining mozzarella and Parmesan cheese.

Bake for 30 minutes or until the top is golden brown. Serve hot.