In a very large skillet, melt butter over medium high heat. Add onion and garlic to skillet and cook, stirring frequently, until onion is softened, about 5 minutes. Add flour, and cook, stirring constantly, for 2 minutes. Add in milk, tomatoes, and tomato sauce; cook for 5 to 6 minutes or until slightly thickened. Stir in chopped chicken, 3 cups of the cheese, salt, pepper, and sugar. Stir in cooked pasta. Pour all into casserole dish and sprinkle the top with remaining cup of cheese.