Rinse the sushi rice under cold water until the water runs clear to remove excess starch.
Cook the rinsed rice in a rice cooker or saucepan by adding 1 cup of water. If using a saucepan, bring the water to a boil, then cover and simmer for about 20 minutes until the rice is cooked and the water is absorbed.
While the rice is still warm, gently fold in sushi vinegar or a mixture of rice vinegar, sugar, and salt. Allow the seasoned rice to cool to room temperature.
Lay out a sheet of Nori seaweed on a bamboo sushi mat with the shiny side facing down. Spread a thin layer of sushi rice evenly over the Nori, leaving a small gap at the top for sealing.
Place the imitation crab, Masago, and avocado slices horizontally across the center of the rice-covered Nori sheet.
Using the bamboo mat, firmly roll the sushi from the bottom edge up, tucking the ingredients inside the roll. Apply gentle pressure to ensure a tight and compact roll.
Moisten the exposed edge of the Nori sheet with water and press it onto the sushi roll to seal.
Use a sharp, moistened knife to cut the sushi roll into 8 equal pieces. Wipe the knife clean and wet it again between cuts for clean, neat slices.
Arrange the Masago sushi pieces on a serving plate and serve with soy sauce for dipping, if desired.