Rinse the Sushi Rice under cold water until the rinse runs clear, then drain and soak the rice for about 30 minutes. Add the drained rice to a pot or rice cooker with the measured Water and place a piece of Kombu on top before cooking; cook according to your rice cooker instructions or bring to a simmer, cover and cook until done, then let rest for 10 minutes and discard the kombu.
While the rice is cooking, warm the Rice Vinegar with the Sugar and Salt in a small saucepan or microwave just until the sugar and salt dissolve; cool slightly.
Transfer the hot cooked rice to a large, shallow bowl. Pour the vinegar mixture evenly over the rice and fold gently with a rice paddle or spatula until the grains are glossy and slightly sticky; fan or blow on the rice as you fold to cool it quickly.
Slice the Tuna, Salmon, and White Fish into bite-sized sashimi pieces suitable for topping bowls, and keep chilled until ready to assemble.
Cut the Tamagoyaki into strips and julienne the Cucumber. Lightly toast the Nori if you like and cut it into thin strips or small pieces.
Divide the seasoned rice among serving bowls and arrange the sashimi pieces on top. Add the Tamagoyaki strips, Cucumber, and pieces of Nori for texture and contrast.
Spoon a few pearls of Ikura over each bowl, and garnish with torn or thinly sliced Shiso leaves and a few slices of Pickled Ginger.
Serve each bowl with a small dish of Soy Sauce and a dab of Wasabi on the side so diners can season to taste. Enjoy immediately for best texture and flavor.
