Pass the fish fillet through a meat grinder, add the egg, flour, salt and pepper to taste. Mix well.
Finely chop the mushrooms and onions for the filling. Sauté the onion in vegetable oil until soft.
Make a tortilla out of minced meat and put it on breadcrumbs, put the filling in the middle.
Fold the cake in half and flatten a little. Bread on all sides and fry in vegetable oil until golden brown.
Serve with a side dish or just with a salad.