4 c. thinly sliced potatoes
3 tbsp. butter
1/4 c. flour
2 tbsp. prepared mustard
1 tsp. salt
1/8 tsp. pepper
2 c. milk
1 (7 oz.) can solid tuna
1 c. thinly sliced onion

Boil potatoes in salted water for 10 minutes. Melt butter; blend in flour, seasonings and milk, gradually. Bring to a boil.

Layer potatoes, onion and tuna in a casserole. Pour sauce over top.

Bake at 375°F for 45 minutes.