Gwen's Old-Fashioned Potato-Beef Casserole

INGREDIENTS

• 1/2 onion, chopped
• Vegetable oil, to coat pan
• 2 cups milk
• 4 tablespoons (1/2 stick) butter
• 2 cups grated sharp Cheddar
• 1 pound lean ground beef
• 1/4 cup all-purpose flour
• Black pepper
• 3 pounds red potatoes, peeled and sliced 1/4-inch thick
• Salt
• 1/2 cup unseasoned dry breadcrumbs