Step 1 - Wash the tomatoes and remove a lid with a knife. Carefully hollow out the inside of the tomato with the help of a spoon. Turn the tomatoes upside down to drain all the juice inside. Set aside.
Step 2 - Separately, open the can of tuna, filter the juice, and in a bowl crumble the tuna meat. Add the onion and parsley, then season with salt, lemon juice and oil. Mix well.
Step 3 - Completely fill the reserved tomatoes with the tuna mixture.
Step 4 - Assemble on a plate with lettuce leaves, hard-boiled egg, olive and mayonnaise.
Step 5 - Optional: the tomato pulp can be chopped and incorporated into the tuna mixture for the stuffing.