Peel and chop the onion and garlic. Cut the monkfish fillets into medallions weighing approximately 50 g each and pound them lightly.
Season the medallions with 1 teaspoon of salt and lemon juice, then coat them in flour. Heat the oil in a high-rimmed pan over medium heat. Fry the medallions for 2 minutes on each side. Remove and keep warm.
In the same pan, sauté the onion and garlic over medium heat for 1-2 minutes. Deglaze with the white wine and add the tomato sauce. Bring to a boil, then stir in the parsley, cayenne pepper, and cream.
Place the fish medallions in the sauce and cook over low heat for 5 minutes. Remove the fish and keep warm. Stir the cognac into the sauce, add the remaining salt to taste, and bring to a boil. Remove from heat and fold in the cold butter in flakes.
Place the fillets on preheated plates and cover with the sauce.