Step 1

Place 2 cups sushi rice in a large bowl and pour in cold water to cover by 2"–3". Gently swish around with your hands to rinse, then drain. Repeat process until water is almost clear (about 3 more times). Pour cold water over rice to cover by 1" and let soak 30 minutes (this will help the grains cook more evenly). Drain well in a sieve.

Step 2

Combine rice and 2½ cups water in a medium saucepan and bring to a boil over high heat, stirring occasionally. Reduce heat to low, cover, and simmer rice until water is absorbed, 12–14 minutes. Remove from heat and let sit (still covered) 15 minutes. This will allow rice to steam and become tender. Uncover and gently fluff rice with a fork. Let sit until cool enough to handle but still warm.

Step 3

Meanwhile, cut three 8½x7½" toasted nori sheets into 3x1¼" strips with kitchen shears; set aside.

Step 4

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.

Step 5

Place 1 cup water in a small bowl and stir in ¾ tsp. Diamond Crystal or ½ tsp. Morton kosher salt. Dip your hands in the salted water, then scoop a little less than ⅓ cup rice into 1 hand. Form into a thick disk 3"–4" in diameter (keep remaining rice in pot covered). Create a small indentation in the center and add 1–2 tsp. filling of choice (or a whole umeboshi). Mold rice up and over filling to create a sphere (filling should not be visible), then press firmly with index finger and thumb to form into a triangle. Place on cutting board or a baking sheet. Repeat with remaining rice and filling, dipping your hands in salt water each time. (You’ll have extra tuna—save it for making a tuna salad sandwich or spooning onto crackers.)

Step 6

Place ¼ cup furikake or toasted sesame seeds on a small plate. Wrap bottom of each onigiri with a strip of reserved nori, then roll remaining exposed sides in furikake.

Step 7

Do ahead: Onigiri can be made 2 days ahead. Tightly wrap and chill.