Roast the buckwheat in a dry, wide pan with a lid for 5 minutes while stirring. At the same time, bring 3/4 l salted water to the boil in a saucepan, pour over the buckwheat and cook covered for 5 minutes over low heat. Remove from the stove and let soak for 10 minutes.
Peel and dice the onions. Wash the spring onions and cut the white end and the greens into rings separately. Carefully clean the oyster mushrooms and mushrooms with kitchen paper or a mushroom brush and cut into 1/2 cm thick slices. Wash and chop the parsley.