Heat the water to boiling and scalding shrimp for 2 minutes. Drain them and dry them gently using paper towels. Cut the flaps of tomatoes and drill them. Lightly salt them inside and let them absorb the salt for about 5 minutes. Grind in a mortar the garlic cloves with a little salt and add the mayonnaise and ketchup, chopped onion and ground pepper. Add sauce thus obtained to shrimp. Drain tomatoes obtained after salting their juice and fill them with shrimp. Use lettuce leaves as a basis for serving.