Boil buckwheat. Peel the onion and mushrooms, finely chop. Fry onions in vegetable oil until golden brown, add butter, heat for 1 minute, add mushrooms. Cook, stirring, for 5 more minutes. Add chopped eggs and buckwheat porridge, salt, heat for 2 minutes, remove from heat. Clean, gut, rinse and dry the fish; rub with salt, fill with stuffing and sew up. Brush with oil, sprinkle with breadcrumbs and bake for 45-60 minutes. in the oven at 200ºС. Serve, cut into portions, garnished with dill.