Favorite Macaroni and Cheese

Ingredients

• 1 1/2 cups milk
• 1 thin slice onion
• 1/2 teaspoon salt
• freshly ground pepper to taste
• 1/4 teaspoon dry mustard
• 2 tablespoon flour
• 1, 8 ounce package Cheddar cheese, shredded
• 1 cup elbow macaroni
• 2 tablespoons butter