Preheat the grill
Gently heat the baked beans in a small pan on the hob
Mix the tuna with the mayonnaise and chives and season to taste
Toast the bread on one side, then turn over and very lightly toast the other side
Spread the tuna mixture evenly over each slice of toast (right up to the edges)
Spoon the baked beans over the tuna, sprinkle with cheese and cook under the grill until the cheese is melted and lightly browned
Add the tomato slices and cook for just 1 minute
Serve hot.