Pollack or haddock can be substituted for the cod in this dish, if you like.
- 3 cups milk
- 1 small leek (white part only), chopped
- 1 small onion, chopped
- 1⁄2 rib celery, chopped
- 3 large sprigs parsley, plus 1 tbsp. chopped parsley
- 3 black peppercorns
- 1 bay leaf
- 1 1⁄2 lb. skinless, boneless cod filets, cut into 2" pieces
- 1⁄2 lb. skinless, boneless hot-smoked cod filets, cut into 1 1⁄2" pieces
- 2 lb. russet potatoes, peeled and quartered
- 12 Tbsp. butter
- 1⁄2 cup heavy cream
- Freshly ground white pepper
- 1⁄3 cup flour
- 1 tsp. prepared English mustard, preferably Colman's
- 3⁄4 cup grated mild white English cheddar
Put milk, leeks, onions, celery, parsley sprigs, peppercorns, and bay leaf into medium pot and bring to a boil over medium-high heat. Remove from heat, cover, and let infuse for 20 minutes. Strain milk, discarding solids, and return milk to pot. Add fresh and smoked cod, cover, bring to a simmer over medium heat, and simmer until cooked through, 7–8 minutes. Strain, reserving milk. Transfer cod to a bowl and cover.
Meanwhile, cook potatoes in large pot of salted boiling water over medium heat until tender, about 25 minutes. Drain potatoes, return to pot, and set aside to let dry out for about 20 minutes. Add 8 tbsp. of the butter, cream, and salt and pepper to taste and mash coarsely. Cover pot and set aside.
Preheat oven to 400°. Melt remaining butter in medium pot over medium heat. Add flour and stir with a wooden spoon until slightly dried out, but not golden, about 4 minutes. Remove pot from heat and gradually add reserved milk, whisking constantly, until smooth. Return pot to heat, add mustard, 1 tsp. of the chopped parsley, and salt and pepper to taste. Cook, stirring, until thickened, 5–7 minutes. Transfer sauce to a 3-quart casserole dish and stir in cod.
Spread potatoes over top of cod mixture; top with cheese. Bake until hot, about 20 minutes. Preheat broiler; broil until top is golden, 2–3 minutes. Scoop pie onto plates and garnish with remaining parsley.