In a saucepan place 3 cups cold water with 2 cups sushi rice. Bring to a boil. Reduce heat, cover and simmer for 20 minutes. Do not lift the lid during cooking.
Prepare seasonings and fillings while rice is cooking.
Aim to have all the ingredients prepared and placed on separate plates and bowls ready to roll when the rice has been cooked and cooled.
In a small bowl, mix together the 4 tablespoons rice vinegar, 3 tablespoons sugar and 1/2 teaspoon salt. This will be used to season the rice.
In a slightly larger bowl (large enough to dip your hands in), mix 1 cup of water with 2 tablespoons rice vinegar. This will be used to keep your hands and equipment moistened, and thus keeping the rice from sticking to everything, while you are making the rolls.
Sprinkle a large tray or platter with this water and rice vinegar solution. This will be used for cooling the rice.
Peel and slice the avocado. Squeeze the juice from the lemon over the slices to prevent them from browning.
Slice the carrots and cucumber into matchsticks. Slice the mushrooms.
Season and cool the rice:.
When the rice has cooked add the sugar, vinegar and salt mixture to the saucepan with the rice and gently stir through. You can buy special rice paddles for this purpose, but a spoon or fork will suffice - just be careful not to break up the grains.
Place the rice onto the moistened tray or platter. Spread it out. Take a magazine, piece of cardboard or similar and fan the rice with it (as you would you face on a hot day). Turn the rice occasionally and continue fanning until the rice has cooled to around room temperature.
Make your rolls:.
Position your bamboo rolling mat so as to roll away from you. Place one sheet of nori on your mat, shiney side down.
Dip your hands in the water and vinegar solution. Take a ball of rice (about 1 cup) and place on the nori. Spread the rice by patting it over the 3/4 of the nori closest to your body. The rice should be around 6 mm (1/4 ) thick. Neaten the two edges of the rice that will be seen when the roll is finished. The 1/4 of the nori without the rice will be the section that seals the roll.
Along the centre of the rice place your fillings.
Now start rolling. Lift the edge of the mat closest to you. I use the mat to just start the roll, but you can use the mat for the whole of the rolling process. Once the roll is started, I pull back the mat and finish rolling (reasonably firmly) by hand, keeping the fillings tucked in as I go. Then I wrap the mat around the roll, and, with firm, even pressure, I run my hands along the mat, thus creating an even and well sealed roll. If the roll doesn't seal completely, I use a finger dampened in the water and rice solution to help it.
Using a very sharp knife damped in the water and vinegar solution cut the rolls into the size desired - in half for longer rolls like those sold at many take away shops, or into 6 or 8 pieces for the smaller servings as shown in the photograph.
I like to refrigerate my rolls for a little while as I like them cold and I find the nori tends to become less chewy. You can, however, eat them straight way. Serve with a little GF soy sauce or tamari and enjoy.
Makes ten 10 cm (4 ) long rolls, thirty 3 cm (1 1/3 ) long rolls or forty 2.5 cm (1 ) rolls.