4 c. thinly sliced pared potatoes
3 tbsp. butter
1/4 c. flour
2 tbsp. prepared mustard
2 c. milk
1 tsp. salt
1/8 tsp. pepper
1 (7 oz.) can tuna, flaked (1 c.)
1 c. thinly sliced, peeled onions

Cook potatoes in boiling salted water for 10 minutes; drain. Meanwhile, melt butter in top of a double boiler over direct heat. Stir in flour, then mustard. Add milk, salt and pepper and stir over boiling water until smooth and thickened. Arrange potatoes, tunas and onions in alternate layers in a greased 2 quart casserole. Pour sauce over all and bake in a moderate oven at 350 degrees for 45 minutes. Makes 6 servings. Another can of tuna may be added for a richer dish.