- 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 of the mayonnaise, capers, lemon juice, mustard, salt and pepper.
- Fill the tomatoes with the tuna mixture.
- Spread the remaining mayonnaise on top, and garnish with parsley leaves.
Serve chilled or at room temperature.