1. Heat the olive oil in a large saute pan, then add onions and garlic. Saute until translucent.
2. In a separate bowl, mix the onions and garlic with the tuna. Add spicy mustard (instead of mayo) until mixture is a spreadable consistency. Add the dill, and mix in well.
3. Spread on a piece of bread, top with slice (or slices) of cheese, and another slice of bread. Toast it lightly in the saute pan, turning once. Serve warm.
4. Cook's Notes: For a healthy choice, use tuna packed in water. This is really good with soup - try a cream of potato, with some extra crushed garlic, and a little bit of dill.