Peel potatoes and boil until tender in salted water for about 20 minutes. Fillet wash, dry, drizzle with lemon juice, salt and pepper. Melt the butter in a skillet, saute 1 tablespoon flour, pour, stirring, milk, cook for 10 minutes. In the remaining flour breaded fish fillets and fry in vegetable oil until golden brown. In the milk sauce add the mustard, salt and pepper. Serve the cod with potatoes and sauce, sprinkle with parsley.