Rinse the cod fillets and dry them with paper towels. Drizzle with lemon juice on both sides, pepper, salt and lightly dust with flour. Heat the butter in a pan and fry the fillets until golden brown on both sides for about 6 to 8 minutes.
For the sauce, put the butter and mustard in a saucepan and melt while stirring. Pour in the cream and season with cayenne pepper, salt and sugar. Let the sauce boil, then reduce the heat a little. Stir frequently with the whisk.