Rinse rice in cold water until clear.
Cook rice using a rice cooker or stovetop.
Heat vinegar, sugar, and salt until dissolved. Cool to room temp.
Mix vinegar mixture into hot rice using a folding motion.
Fan rice while mixing for a glossy texture.
Cut cucumber and avocado into thin strips.
Slice fish into ½-inch strips if using.
Place nori on a bamboo mat, shiny side down.
Spread rice over nori, leaving 1-inch at the top.
Sprinkle sesame seeds over rice.
Arrange fillings in the center of the rice.
Roll using the bamboo mat, applying gentle pressure.
Wet the edge of nori to seal the roll.
Let the roll rest for 1-2 minutes before cutting.
Wet a sharp knife and cut the roll into 6 pieces.
Serve with pickled ginger, wasabi, and soy sauce.