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.
Wow! Have just cooked this, was gorgeous! Mind you, I did cook it for two people (and 2 fillets of cod), but didn't halve the quantity of sauce - naughty, but nice! I served it with rice and steamed french green beans. Only changes I made - in the initial 10 minutes of cooking the sauce, I added a bay leaf and a couple of parsley stalks to add flavour, but removed them before adding the fresh herbs. I also added a pinch of ground nutmeg to the sauce. Will definitely cook this again!
Wonderfully delicious. That sauce can get addicting; the fresh herbs added right at the end make all the difference. I do recommend this recipe!