Wash and dry the champignons. Peel the red onion. Prepare the remaining ingredients.
Prepare the rice. For onigiri it is prepared in the same way as for sushi. Rinse the rice 3-4 times until the water is more or less clear and place in a sieve. When all the water has drained, transfer the rice to a small ladle or pan, pour a glass of cold water and put on fire. When the water boils, reduce the heat to low and cover the rice with a lid. Cook rice for 10-12 minutes. When all the water has been absorbed into the rice, remove it from the heat and leave to steam for another 10 minutes. Prepare the dressing: mix vinegar, salt and sugar. Pour the resulting dressing over the rice and stir. Cool it down. The onigiri rice is ready.
Now start filling. Slice half a red onion into thin quarter-rings. Cut the champignons into thin slices or small cubes.
Heat vegetable oil in a frying pan. Fry the onions and mushrooms over medium heat for 7 minutes. Add a tablespoon of soy sauce, stir and cook until it evaporates
Transfer the sautéed onions and mushrooms to a small bowl. Add a tablespoon of sour cream and stir.
Now you can start shaping the onigiri themselves. For this we will use a special triangular mold. If you don’t have one, then you can sculpt onigiri simply in your hands. Take a small amount of rice, approximately 60-70 g, and place it on the bottom of the mold, forming small sides. Place 50 g of filling into the resulting cavity.
Place another 70 g of rice on top of the filling. Press the lid of the mold onto the rice and press down the onigiri. Form all products in this way. You should have about 2-3 pieces. Cut half a sheet of nori into several pieces. Wrap a piece of nori around each onigiri as shown in the photo.
Place onigiri on a plate and sprinkle with black and white sesame mixture. Ready. Bon appetit!
Serve the onigiri with soy sauce.