Combine rice with water in a medium saucepan and bring to a boil over high heat. Reduce heat to low, cover, and simmer for 15 minutes until rice is tender and water is absorbed. Remove from heat and let sit, covered, for 10 minutes. Fluff with a fork.
Meanwhile, in a small bowl, mix rice vinegar, sugar, and salt until dissolved. Pour this mixture over the hot cooked rice and gently fold to combine. Let the rice cool slightly.
Prepare vegetables by slicing cucumber, avocado, and carrots into thin or bite-sized pieces. Arrange them on a serving plate for toppings.
1 cup sushi rice
Slice sashimi-grade fish or cooked seafood thinly and set aside. Arrange the slices over the rice or the prepared vegetables.
1 cup sushi rice
To assemble the bowl, scoop a generous portion of seasoned rice into each serving bowl. Top with sliced vegetables and fish, then drizzle with soy sauce and sesame oil. Garnish with sesame seeds and serve with pickled ginger.
1 cup sushi rice