Macaroni and Cheese

3 cups cooked macaroni
2 tablespoons butter or margarine, melted
1 1/3 cups evaporated milk
1/2 teaspoon salt
1 to 1 1/2 cups shredded sharp process cheese
2 tablespoons minced onion

Toss macaroni with butter or margarine. Add all remaining ingredients. Pour into lightly greased crockette. Cover and cook for 4 to 6 hours, stirring once or twice.