Preheat 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 onions, garlic, and dried basil and stir to combine. Continue to cook for 5 minutes.

Add mushrooms and stir to combine. Continue to cook for 5 minutes.

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

Stir in spinach and remove from heat.

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

Whisk in milk and cook for 5 to 10 minutes, whisking constantly. Once mixture is thickened, stir in ¾ cup mozzarella cheese and salt.

In a large pot, bring water to a boil. Cook spaghetti for 5 minutes. Drain. Add to meat sauce and stir.

Spread meat mixture into an 9x13 inch casserole dish. Pour cheese sauce over top. Sprinkle with remaining mozzarella cheese and Parmesan cheese.

Bake for 30 minutes or until topping is golden. Serve hot.