Prepare the fillings. Mix one 5-oz. can tuna, 2 Tbsp. mayonnaise, preferably Kewpie, and 1 tsp. soy sauce in a small bowl to combine. Place 4 umeboshi on a cutting board. Working one at a time, press a chef’s knife against fruit until pit is exposed; pull out and discard. Set fruit aside (no need to chop). Working over the sink and using your hands, squeeze liquid out of ⅓ cup kimchi; chop into small pieces.