Creamy parsley sauce -- here, over cod -- is an old English favorite. The herb is stirred in at the end, so it maintains its vibrant flavor and color. This main dish is delicious served with boiled potatoes or rice.
Melt butter over medium heat in a small saucepan. Add flour; whisk until a paste forms. Reduce heat to low; cook, whisking, 2 minutes. Whisking constantly, pour in milk in a slow, steady stream; whisk until incorporated. Whisk in stock. Add 1/2 teaspoon salt, and cook, whisking occasionally, 10 minutes. Remove from heat; let stand 10 minutes to thicken slightly. Stir in parsley and chives.
Meanwhile, bring 1 1/2 quarts water, the lemon peel, 1/4 cup lemon juice, and 1/2 teaspoon salt to a boil in a medium saute pan or other wide straight-sided pan. Place fillets in liquid (do not crowd). Return to a boil, then immediately turn off the heat. Let stand until just cooked through, about 10 minutes.
Reheat sauce over medium-low heat, if necessary, and add remaining 2 teaspoons lemon juice. Gently lift fillets from liquid with a slotted spatula; place a fillet on each of four plates. Spoon one-quarter of sauce over each serving, and garnish with a parsley sprig.