- Bring a large pot of salted water to a boil.
- Add potatoes and cook until tender but fi rm.
- Preheat oven to 300 degrees.
- In another pan, combine mustard, fl our, milk, salt, and soup. Mix well and heat until thickened.
- Starting with potatoes, arrange potatoes, tuna, and onions in alternating layers in a 9x13-inch baking dish. Pour the mustard sauce over the top.
- Bake in a preheated oven for 1 hour or until bubbly
- Optional: sprinkle a layer of cheddar or mozzarella cheese over the top.