Prepare the hand water by combining the water and rice vinegar in a medium size bowl.
Place the bamboo mat on your work surface with the sticks running horizontally.
Put the nori sheet on top with the rough side facing up.
Lightly moisten the palm of one hand with hand water and rub your hands together to prevent the rice from sticking to them.
Place the rice in the center of the nori sheet and use your hands to spread the rice evenly over the sheet leaving ½ inch along the far side. Be careful not to compress the rice too much and use the hand water often to keep your hands moist.
Flip the nori sheet over so the rice is face down.
Place the sliced tuna and avocado into the center of the nori
Put your hands on both sides of the mat and start to roll.
Once you get to the end hold firmly to tighten the roll.
Place the finished roll on a wooden board.
Sprinkle toasted sesame seeds on top.
Place the tip of your knife in the hand water and lift to wet the whole surface of the blade.
Slice the roll in half, smoothly and quickly. Then place the halves opposite each other and slice into thirds to create six pieces.
Remove any rice that starts to stick to the knife and remoisten with the hand water regularly to prevent the rice from sticking.
Serve with soy sauce and sushi ginger.