Stir the ingredients for the dressing together (mayo through capers) and set aside.
Split the buns in half and toast if you like. Slather some of the dressing on both the tops and bottoms of the buns.
Put half the tuna (1 can per sandwich) on the bottom portions of the buns, followed by half of the radish slices, hard cooked egg slices, olives, onion, green beans and romaine lettuce. Finish with a few dashes of sea salt and/or black pepper.