Salt and pepper the fish fillet. Then drizzle with lemon juice and turn in flour. Brown well in a little fat and remove from the pan. Brown the onion pieces in a little fat, then add the stock and cream. Stir in the tarragon mustard and simmer until the sauce thickens a little. Season to taste and, if necessary, season with salt and pepper. Now put the fish in the sauce and let it steep for a moment. Arrange and serve on plates.