Preheat the oven to 190 ° C. Rinse the carcass of fish, dry, cut into portions. Grate with salt and freshly ground pepper. Pieces of fish to lay on the bottom of the heat-resistant dishes in one layer.
Chop onions in half rings, finely chop the mushrooms. Heat butter and vegetable oil in a small pan. When the butter stops foaming, add onions and fry until soft. Add mushrooms, salt and pepper to taste and fry, stirring, 4-5 minutes.
Pour cream on the mushrooms and mix well. Pour fish over the resulting sauce. Spread the mushrooms evenly over the entire surface. Put in a preheated oven and oven for 20 minutes.
After the specified time, first sprinkle the fish with parsley, and grated cheese on top. Bake another 10 minutes, the cheese should be well browned. Remove from oven and serve immediately.