2 cups Japanese short-grain rice
3 teaspoons umeboshi (pickled plum) paste
3½ oz silken tofu, mashed
1 tablespoon finely chopped shiso leaves
1 teaspoon sesame oil
6 whole shiso leaves
2 tablespoons toasted sesame seeds
Wash the rice under cold running water. Put the rice and 2¾ cups cold water in a small, heavy-based saucepan and bring to the boil over high heat. Cover with a tight fitting lid, reduce the heat to low and cook for 15 minutes. Remove from the heat, fluff the rice with a spatula and cover with a clean towel. Set aside for 15 minutes.
Combine the umeboshi paste, tofu and shiso in a bowl.
Line a teacup with plastic wrap so that it hangs over the side. Brush the plastic wrap with water. Gently press 1 loosely packed cup of rice into the teacup. Use a finger to make an indentation in the centre of the rice and put 2 teaspoons of the tofu mixture into the indentation. Draw the sides of the plastic wrap over the rice, remove from the teacup and firmly twist to enclose the rice. Keep twisting until the rice forms a ball. Repeat this process to make another five rice balls. Leave the rice wrapped in the plastic and set aside for 30 minutes.
Preheat the broiler and line a baking tray with parchment paper.
Unwrap the rice balls and lightly brush with the sesame oil. Place on the baking tray and broil, turning often, for about 2–3 minutes, or until the rice is just turning golden.
To serve, place each onigiri on a shiso leaf and sprinkle with the sesame seeds.