Au gratin potatoes

8 cups cubed, peeled potatoes
1/4 cup butter or margarine
2 T. all-purpose flour
3/4 tsp. pepper
1 lb. processed american cheese, cubed
minced fresh parsley (optional)

In a large sauce pan, cook potatoes in boiling water until done. Drain and place in a greased 2 1/2 - quart baking dish. In a sauce pan, melt butter. Add the flour, salt and pepper. Stir to form a smooth paste. Gradually add milk, stirring constantly. Bring to a boil. Boil & stir for 1 minute. Add cheese. Stir just until melted. Pour over potatoes. cover and bake at 350 F for 45-50 minutes or until bubbly. Sprinkle with parsley.