Sprinkle the cod fillets with lemon juice, salt and pepper. Peel onions and cut into small cubes, fry gently in the melted butter until transparent. Fry fillet of cod for half a minute on each side. Reduce the heat, pour in the wine, simmer 4-5 minutes. Remove the fish, add to the pot the cream, mustard, stir and bring to a boil, add the strained capers, salt and pepper. Serve fish with sauce, lemon slices and dill.