Soak the shiitake in hot water to cover for 10 to 20 minutes. Reserve soaking water. Remove stems from shiitake, discard, and slice caps. Place in a saucepan with 1 cup water, including the shiitake soaking water, and bring to a boil. Reduce flame to low, add shoyu, cover and simmer 20 minutes. Remove shiitake and place in a small bowl. The shiitake soaking water may be chilled and used as a dipping sauce for the sushi.
Thoroughly mix the vinegar in with the cooked rice, fluff to cool, if the rice is hot. The best sushi is made with room temperature or leftover rice. Hot rice causes the nori to fall apart easily.
Take a bamboo sushi mat, set it on the counter or cutting board. Place a sheet of sushi nori on the mat with the rough, dull side facing up. Take 1 1/4 to 1 1/2 cups of cooked rice and spread it evenly on the bottom half of the sheet of nori. Moisten a wooden spoon or rice paddle and press down firmly on the rice to evenly spread and cover half the sheet of nori. Place 1/4 of the shiitake, in a straight line, across the center of the rice (horizontally). Place 3 or 4 slices of avocado on top of the shiitake, in a straight line. Pull the mat up with your thumbs, hold and tuck avocado and shiitake with all eight fingers. Roll forward in a jelly roll fashion, and tuck the nori forward, while pulling back the mat slightly each time you roll forward. Just before you reach the end of the sheet of nori, moisten the end with a little cold water. Continue rolling until you have a cylinder shaped roll. Tightly wrap the bamboo mat around the roll, and squeeze gently to seal the nori roll. Leave the mat around the roll. To make the ends of the roll even, with your hands, take a little rice and place it in each end, tapping lightly with a wooden pestle or spoon handle to pack the rice firmly into the ends. Remove the mat and place the roll on the cutting board.
Moisten a sharp knife with water and cut the roll in half. Cut each half in half, then each quarter in half, moistening the knife each time before cutting. You should now have eight equal size pieces of sushi. Repeat the above steps until you have 6 rolls or 48 pieces of sushi.
Try mixing a little Eden Wasabi Powder (Japanese Mustard) in with the shiitake cooking water before chilling and using as a dip sauce.
Per serving - 428 calories, 8 gram fat (15% calories from fat), 15 gram protein, 78 gram carbohydrate, 27 gram fiber, 0 milligram cholesterol, 184 milligram sodium