"To take a tuna melt from good to great, it has to have all the right components. In other words, plain mayo alone won't cut it. Here's how we do it: open-faced with crusty toasted bread, melty cheese, capers, cornichons, Dijon mustard, and plenty of seasoning. On a chilly fall day, nothing tastes better."
Set the oven to broil and move the rack to the highest position.
In a medium bowl, combine tuna, mayonnaise, mustard, red onion, capers, cornichons, lemon juice, thyme, salt, and pepper and stir thoroughly until combined. Leave the tuna in slightly large chunks, if possible.
Arrange bread on a baking sheet and spread mayonnaise on each slice. Divide tuna salad evenly among slices, then top each with 2 slices of tomato and 1 slice of cheese.
Broil tuna melts until cheese is golden brown and bubbling, 3 to 4 minutes. Serve warm.