Creamy Cod and Potato Gratin

Instructions

• Place potatoes in a large pot of well-salted water and bring to a boil over high heat. Reduce heat and simmer gently until tender, about 20 minutes.
• While potatoes simmer, trim, bone, and cut the cod fillets across into 2-inch pieces.
• Drain potatoes well and return to the pot. Add a pinch of cayenne, salt, and 4 tablespoons butter. Mash until smooth.
• Preheat the oven to 450 degrees F (230 degrees C). Grease 2 individual-sized casserole or baking dishes with 1 tablespoon butter. Place both dishes on a baking sheet.
• Transfer about 2/3 cup of the mashed potatoes into each of the prepared dishes. Smooth out evenly and make a shallow depression with a spoon down the middle. Set aside.
• Heat remaining 2 tablespoons butter in a deep 10-inch skillet over medium-high heat. Cook the shallots in the hot butter with a pinch of salt until they just start to turn golden, 3 to 4 minutes. Add cream, lemon zest, and a pinch of cayenne, and bring to a simmer. Add cod and wait for the sauce to come back to a simmer. Cook for 1 minute, while basting with the sauce; turn and cook for 1 minute more.
• Pour in lemon juice and turn off the heat; stir for about 30 seconds.
• Divide the pieces of cod over the potato. Add enough sauce to just cover the fish and return the pan to the stove.
• Bring sauce to a boil over medium-high heat and cook until the sauce reduces and thickens, 1 to 2 minutes. Turn off the heat and stir in tarragon.
• Spoon the thickened sauce over the cod. Add a light dusting of Parmigiano-Reggiano cheese.
• Bake in the center of the preheated oven until cod is cooked through and the sauce is browned and bubbling, 10 to 15 minutes. Let rest for 5 minutes before serving.