1. Boil and skin the fillets.
2. chop the onions finely and break the egg into it, beating lightly all the while.
3. Add the French mustard, sauce, salt and pepper and mix with the fillets.
4. When the batter becomes smooth enough,  make round cakes with your palms, and saute in 2 tbsps oil.

(This should be done in a nonstick frying pan, and the cakes should be cooked on low heat)