Cook the rice: Make 2 cups of warm, freshly cooked short-grain rice. If using leftover rice, reheat with a splash of water to bring back softness.
Mix the tuna: In a bowl, combine drained tuna, mayonnaise, mustard, onion or scallion, and lemon juice.
Season with salt and black pepper. It should be creamy but not runny.
Make the soy drizzle: Stir together soy sauce, sugar, mirin, rice vinegar, grated garlic, and gochugaru until the sugar dissolves.
Assemble the bowl: Add warm rice to a bowl. Drizzle sesame oil over the rice and fluff with chopsticks or a fork.
Top it off: Spoon the tuna-mayo mixture over the rice.
Add chopped kimchi if you’re using it. Pour the soy drizzle over the top, a little at a time, to taste.
Finish with texture: Sprinkle toasted sesame seeds and crumbled roasted seaweed. Add a soft-boiled or fried egg for extra richness.
Eat right away: Mix the bowl lightly so you get a bit of everything in each bite.