Liberally season the fish on both sides with salt and pepper. In a large frying pan, melt the butter over medium heat until it is foaming. Add the fish and saute for 2 minutes on each side. Transfer to a warm plate. Reduce heat to medium-low, add the wine and leeks, and saute until the leeks are soft, about 5 minutes. Stir in the cream and mustard. Add the fish to the pan, baste with the sauce, cover, and cook over medium-low heat until still translucent in the center, about 5 minutes. The fish will continue to cook through off heat. Serve topped with sauce.