How to Make It
Combine 11/4 cups water and rice in a medium saucepan over medium-high heat. Bring to a boil; reduce heat to low, cover, and cook 15 to 20 minutes or until liquid is absorbed (do not uncover while rice is cooking). Remove pan from heat; let stand covered 10 minutes.
While rice stands, combine 2 tablespoons rice vinegar and sugar in a small microwave-safe bowl and microwave on HIGH 30 to 45 seconds or until boiling; stir until sugar dissolves. Gently stir vinegar mixture into rice using a rubber spatula.
Whisk together remaining 1 teaspoon rice vinegar, soy sauce, mirin, sesame oil, and red pepper in a medium glass bowl. Add diced tuna to bowl; sprinkle tuna with kosher salt and toss gently to combine with soy sauce mixture. Let stand at room temperature for 5 minutes.
Divide rice evenly between 2 round serving bowls (about 11/4 cups rice per bowl), pressing into an even layer. Divide tuna mixture evenly between bowls (about 2/3 cup tuna per bowl), spooning the tuna over the rice in an even layer across 1/2 of the bowl (to create the red half of the Pokéball). Trim nori strips with kitchen shears to fit inside the serving bowls and place along the dividing line between the rice and the tuna mixture in each bowl, pressing gently to adhere. Place 1 radish slice in the center of each nori strip. Gently spoon black sesame seeds around the edges of each radish. Serve immediately.