4 c. thinly sliced potatoes
3 tbsp. butter
1/4 c. flour
2 tbsp. prepared mustard
2 c. milk
1 tsp. salt
1/8 tsp. pepper
1 can tuna
1 c. thinly sliced onions

Preheat oven to 350 degrees. Cook potatoes in boiling water for 10 minutes; drain. Melt butter on low heat. Stir in flour and mustard. Add milk, salt, and pepper. Stir until smooth and thickened. Arrange potatoes, tuna, and onion in layers in greased casserole. Pour sauce over all and bake 45 minutes.