Rinse the sushi rice under cold water until it runs clear, then drain. Add the rice, water, soy sauce, mirin, rice vinegar, salt, grated garlic and sugar to a small saucepan and stir to combine. Scatter the sliced mushrooms evenly over the top. Bring the pot to a boil, cover with a tight lid, reduce to low, and cook for 12 minutes. Turn off the heat and let it steam for another 12 minutes without lifting the lid. Or just cook as normal in a rice cooker.