Makes 4 open-faced sandwiches; serves 2 to 4
Position a rack about 6 inches from broiler element and preheat broiler.
Combine tuna, mayonnaise, red onion, lemon juice, pepper, salt, and oregano in a medium bowl and stir until thoroughly combined.
Arrange bread slices on a baking sheet and spread additional mayonnaise over each slice, if desired. Divide tuna salad evenly among bread slices, then top with tomato. Arrange provolone evenly over sandwiches. Place baking sheet under broiler and cook until cheese is golden and bubbly, 3 to 4 minutes. Serve hot.