Cook sushi rice with water in a saucepan until boiling, then simmer covered for 20 minutes.

Mix rice vinegar, sugar, and salt in a small bowl for rice seasoning.

In a larger bowl, mix water and rice vinegar for moistening hands and equipment.

Spread the water and rice vinegar solution on a tray or platter for cooling the rice.

Peel and slice avocado, squeezing lemon juice to prevent browning.

Slice carrots and cucumber into matchsticks, and slice shiitake mushrooms.

Add the sugar, vinegar, and salt mixture to the cooked rice and gently stir.

Spread the rice onto the moistened tray or platter and fan until it cools to room temperature.

Position a bamboo rolling mat and place a sheet of nori on it, shiny side down.

Dip hands in the water and vinegar solution and spread a ball of rice over 3/4 of the nori.

Place fillings (avocado, carrots, cucumber, mushrooms) along the center of the rice.

Lift the edge of the mat and start rolling, keeping the fillings tucked in.

Wrap the mat around the roll and apply firm, even pressure.

If the roll doesn't seal, use a finger dampened in the water and rice solution to help.

Cut the rolls into the desired size using a sharp, dampened knife.

Refrigerate rolls for a while before serving or eat immediately.

Serve with gluten-free soy sauce or tamari.