Cut off the top 1/3 part of each tomato. Then carefully scoop out and discard the inside of each tomato with a spoon.
To make the filling: Drain the oil from the tuna and place the tuna in a bowl. Then add the parsley. Chop 2 tablespoons capers and add to the mix. Add 3/4 of the mayonnaise. Mix well. Divide the tuna mixture among the tomato shells. Spoon the mixture into the tomato shells. Top with the remaining mayonnaise and capers.