4 cans (5 ounces each) solid white tuna packed in water, drained
1/3 cup mayonnaise, plus more for spreading
1/4 cup finely chopped red onion
4 teaspoons capers, rinsed and drained
1 tablespoon fresh lemon juice
1/2 teaspoon coarse salt
1 teaspoon ground pepper
1/4 teaspoon dried oregano, crumbled between your fingers
4 slices crusty bread
8 thin slices tomato
4 slices provolone
Heat broiler, with rack in highest position. In a medium bowl, combine tuna, mayonnaise, onion, capers, lemon juice, salt, pepper, and oregano and stir until thoroughly combined.
Arrange bread on a baking sheet and spread a little mayonnaise on each slice. Divide tuna salad evenly among slices, then top each with 2 slices tomato and 1 slice cheese. Broil until cheese is golden brown and bubbling, 3 to 4 minutes.