Slice the tops off the tomatoes and scoop out the seeds and most of the dividing walls inside.
Sprinkle the insides of the tomatoes lightly with salt and invert on paper towels for 1/2 hour to drain the excess moisture.
Mix together the tuna, 1/2 cup (125 ml) of the mayonnaise, capers, lemon juice, mustard, salt, and pepper, and fill the tomatoes with the tuna mixture.
Spread the remaining mayonnaise on top, and garnish with parsley leaves.