3 pounds white or red potatoes, peeled and sliced 1/4-inch thick 1 pound lean ground beef 1/2 cup chopped onion 4 tablespoons butter 1/4 cup flour 1 teaspoon salt 1/4 teaspoon black pepper 2 cups milk 2 cups grated sharp Cheddar cheese 1/2 cup unseasoned bread crumbs
Place potatoes in a large saucepan with water to cover by 1-inch. Add a generous pinch of salt and cook for 15 minutes or until tender. Drain and arrange in a 6x9x2-inch casserole dish.
In a large skillet, combine beef and onion and cook together over medium heat until beef is browned and onion is softened, about 12 to 15 minutes. Pour off any excess fat.
Preheat oven to 350 degrees F.
Melt butter in a medium saucepan and whisk in flour to make a roux. Cook over medium heat, whisking constantly, until mixture bubbles and flour turns light brown in color. Gradually whisk in milk and continue to stir while cooking over medium heat.
When mixture thickens, whisk in salt and pepper, then stir in cheese and browned beef. Pour mixture over potatoes and bake for 20 minutes until heated through and bubbling.
Sprinkle bread crumbs on top and bake until crumbs are toasted, about 5 minutes longer.