Remove the stems from the tomatoes, then make a shallow X-shaped incision on the opposite side. Bring a pot of water to a boil. Gently lower the tomatoes into the water. After about 10 seconds, when the skin begins to peel back, remove them. Transfer to cold water, then peel the skin starting from the loosened area. Remove and drain well.
Drain the canned tuna and flake it if using chunk-style. Finely chop the onion. In a bowl, combine the tuna, onion, and mayonnaise, and mix well.
Cut off the stem end of the peeled tomatoes. Using a spoon, scoop out the inside to about halfway deep, removing some of the flesh.
Fill the tomatoes with the tuna mixture a little at a time, then smooth the surface with the back of a spoon. Place them in a tray or container and chill in the refrigerator for at least 30 minutes.
Mix all the ingredients for the dressing in a small bowl. Plate the tomatoes and pour the dressing over them.