Place the rice in a fine mesh strainer or colander and rinse it under cold running water, stirring gently with your fingers or a spoon. Continue rinsing until the water runs clear, indicating that most of the starch has been removed. (Rinsing helps remove excess starch from the surface of the rice grains, which can result in a cleaner, fluffier texture once cooked. Some sushi rice brands are pre-rinsed or partially rinsed, so check the package for instructions.)

In a large saucepan, bring the water to a boil. Stir in the rinsed rice, then reduce the heat to low. Cover the saucepan and simmer for 20 minutes, or until the rice is cooked and the water is absorbed. Remove from the heat and let the rice steam, covered, for 10 minutes to firm up and absorb any excess moisture.

While the rice cooks, in a small bowl, whisk together the rice vinegar, sugar, and salt until the sugar and salt are fully dissolved.

After the resting period, gently fold the vinegar mixture into the cooked rice until evenly distributed. Do not mash the rice; you want to maintain its fluffy texture.

Transfer the seasoned rice to a baking sheet and spread it out evenly using a rice paddle or spatula. Let the rice cool to room temperature, for about 30 minutes. As it cools, the grains of rice will develop a slightly sticky texture, perfect for rolling sushi.

In a small bowl, whisk together the tamari, hoisin sauce, sesame oil, garlic powder, and ground ginger. Set aside.

Set a medium skillet over medium-high heat. When the pan is hot, add the sliced mushrooms and tamari mixture. Cook for 2-3 minutes, until the mushrooms are tender and golden brown. Remove from heat to cool.

Place a bamboo sushi mat on a flat surface and set a small bowl of water nearby. Position a nori sheet in the center of the mat. Spread a layer of cooked sushi rice on the sheet of nori, leaving a 1-inch border at the top and bottom edges.

Add the cooked mushroom slices along the edge closest to you.

Using the bamboo mat, slowly but tightly, using gentle pressure, roll from the bottom around the mushrooms. Dip your fingers into the bowl of water then lightly seal the nori.

Using a very sharp knife, slice into bite-size pieces and serve.