Preheat oven to 350 degrees. Lightly grease a 13x9 baking dish; set aside.
In a medium pot over medium heat cook ground beef; drain. Add mushrooms, salt and pepper to taste, and cook until mushrooms are tender. Add marinara or pasta sauce; cook 5-8 minutes or until thickened.
Meanwhile, in a large pot over high heat cook rotini in boiling, salted water until al dente; drain. Return to pot and add butter or margarine and stir well until melted. Spread in greased baking dish.
Spoon meat sauce mixture over pasta and sprinkle with cheese.
Bake until cheese is melted and bubbly, about 30 minutes.