Peel and chop the onion and garlic. Cut the monkfish fillets into medallions weighing approx. 50 g and pound them lightly.
Season with a teaspoon of salt and lemon juice and turn in flour. Heat the oil in a pan with a high rim. Fry the medallions in it over medium heat for 2 minutes on each side. Take out and keep warm.
Put the onion and garlic in the hot fat and sauté briefly. Deglaze with white wine and add the tomato sauce. Bring to the boil once and stir in the parsley, pepper and cream.
Place the fish medallions in it and let it steep for 5 minutes over a low heat. Then take it out, keep it warm and stir the cognac into the sauce. Then add the remaining salt to taste, bring to the boil once and then remove from the fire. Fold in the cold butter in flakes.
Place the fillets on preheated plates and cover with the sauce.