Rinse 200 grams of brown rice thoroughly under cold water until the water runs clear. Soak the rice in water for 30 minutes, then drain.
In a saucepan, combine the soaked brown rice with 400 ml of water. Bring it to a boil, then reduce the heat to low, cover, and simmer for 40-45 minutes, or until the rice is cooked and the water is absorbed. Remove from heat and let it stand, covered, for 10 minutes.
In a small saucepan, combine 50 ml rice vinegar, 15 grams of sugar, and 5 grams of salt. Heat the mixture over low heat until the sugar and salt dissolve. Remove from heat and let it cool.
Transfer the cooked brown rice to a large bowl and gently mix in the vinegar mixture using a wooden spatula. Allow the rice to cool to room temperature.
Prepare your desired sushi fillings, such as thinly sliced cucumber, avocado, carrot, and cooked fish or seafood.
Place a sheet of nori (seaweed) on a bamboo sushi mat. Spread a thin layer of the seasoned brown rice evenly over the nori, leaving a small border at the top and bottom.
Arrange your chosen fillings in a horizontal line across the center of the rice.
Using the bamboo mat, roll the sushi tightly, applying gentle pressure to ensure the roll is firm. Wet the top border of the nori with a little water to seal the roll.
Using a sharp knife, slice the sushi roll into bite-sized pieces. Serve with soy sauce, pickled ginger, and wasabi.