Beat the egg lightly in a bowl. Add the buckwheat and mix well.
Roast buckwheat and egg: Heat a nonstick pan without any fat, place the buckwheat into the pan, and cook for about 3-4 minutes until all the corns are dry and separated.
Slice onions: In the meantime, slice the onions thinly.
Simmer buckwheat: Remove the buckwheat from the heat and transfer it to a small saucepan. Add the chicken stock or vegetable broth, bring to a boil, turn the heat down, and simmer for about 15 minutes or until the buckwheat is soft and the stock has been absorbed.
Cook onions: Heat 1 tablespoon butter and the oil in the pan and cook the onions on low heat for about 15 minutes or until they are very soft and deeply golden. Stir often, and don't let them catch. I added two small splashes of water in between; the onions were threatening to catch, and I didn't want to add more butter.
Cook mushrooms: Clean the mushrooms with kitchen paper and slice them as well. When the onions are cooked, add the mushrooms and continue cooking until they release their juices and the juices then evaporate, which will take about 5-7 minutes. Adjust the taste with salt and pepper.
Combine: Add the buckwheat to the pan and stir well to mix. Stir in the second tablespoon butter. Chop the dill and the parsley and add them to the pan. Serve immediately.
Notes
Roasted buckwheat has a darker color than raw buckwheat. You will be able to buy it roasted most of the time, especially if buying it in a Russian store.