Scalloped Tuna - Barbara Seng

4 cups thinly sliced potatoes            
3 tbsp butter
¼ cup flour                                          
2 tbsp prepared mustard
1 tsp salt                                                               
¼ tsp pepper
2 cups milk                                          
1-7oz can tuna
1 cup thinly sliced onion

Cook potatoes in boiling water for 10 minutes and drain.  Melt butter, stir in flour, salt and pepper and cook until bubbly.  Stir in mustard and milk and cook until smooth and thickened.  Arrange potatoes, drained tuna, and onions in a greased baking dish.  Pour sauce over potatoes and bake for 45 minutes at 375º.