Preheat the grill to high.
In a bowl mix together the tuna, onions, mayonnaise, mustard & black pepper.
Toast the bread, cut side up until lightly browned. Spread the tuna mixture on top then scatter  the smoked  cheese over the top. Lay the tomato slices on top of the cheese.
Grill until bubbling and golden.
 Buy Knockanore Smoked Cheese Here