Mom’s Favorite Baked Mac and Cheese

INGREDIENTS

• 2 tablespoons butter
• 1/4 cup finely chopped onion
• 2 tablespoons all-purpose flour
• 2 cups milk
• 3/4 teaspoon salt
• 1/2 teaspoon dry mustard
• 1/4 teaspoon ground black pepper
• 1 (8 ounce) package elbow macaroni
• 2 cups shredded sharp Cheddar cheese
• 1 (8 ounce) package processed American cheese, cut into strips