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.