Tuna Potato Casserole Recipe

Ingredients (9)

• 5 medium potatoes, cooked and sliced
• 3 tablespoons butter
• 1/4 cup flour
• 1 teaspoon salt
• 1/8 teaspoon pepper
• 1 1/2 tablespoons prepared mustard
• 2 cups milk
• 1 can (7 ounces) tuna, drained
• 1 cup chopped onion