3 cups diced cooked potatoes (boil for 20 minutes) You can also used canned potatoes.
6 tablespoons butter
3 tablespoons flour
1 ½ cups milk
1 cup shredded cheddar cheese
Salt and pepper
1 cup bread crumbs  

Place potatoes in shallow baking dish.  In sauce pan melt butter.  Reserve 3 tablespoons and set aside.  To the butter add 3 tablespoons flour.  Stir in until well blended.  Gradually stir in milk.  Continue cooking until smooth and thick.  Add cheese and continue to cook until cheese has melted.  Pour over potatoes, mix gently.  Mix bread crumbs with remaining butter and sprinkle over potatoes.  Bake at 400 degrees for 15 minutes.  Then broil until golden brown.

 
3 cups cooked, peeled, and whipped sweet potatoes
1/2 cup margarine softened
1 cup sugar
2 eggs
1 tsp vanilla

Topping:
1/2 cup margarine softened
1/3 cup flour
1 cup light brown sugar
1 cup chopped pecans

Preheat oven to 350 degrees.  Combine first 5 ingredients and pour into 9x9 greased casserole.  Prepare topping and spoon over top.  Bake for 30 minutes.