Scallop-Topped Potato and Celery-Root Puree with Lemon Brown-Butter Sauce

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Ingredients

1. 1 1/2 pounds baking potatoes (about 3), peeled and cut into 2-inch chunks
2. 1 pound celery root, peeled and cut into 2-inch chunks
3. Salt
4. Fresh-ground black pepper
5. 8 tablespoons butter, at room temperature
6. 1 tablespoon cooking oil
7. 2 pounds sea scallops
8. 3 tablespoons chopped flat-leaf parsley
9. 1 tablespoon drained capers
10. 1 teaspoon lemon juice

1. Put the potatoes and celery root in a medium saucepan of salted water. Bring to a boil, reduce the heat, and cook at a low boil until tender, about 20 minutes.
2. Reserve 1/2 cup of the cooking water, and then drain the potatoes and celery root. Put them back into the saucepan along with 1/4 teaspoon salt and a pinch of pepper. Mash over very low heat, incorporating the reserved cooking water and 5 tablespoons of the butter.
3. In a large nonstick frying pan, heat 1/2 tablespoon of the oil over moderately high heat. Season the scallops with 1/4 teaspoon salt and 1/4 teaspoon pepper. Add half the scallops to the pan. Cook until browned, about 2 minutes. Turn and cook until browned on the second side and just done, about 1 minute. Remove. Heat the remaining 1/2 tablespoon oil in the pan and cook the remaining scallops. Remove.
4. Reduce the heat to moderate. Add the remaining 3 tablespoons butter to the pan and cook until golden, about 2 minutes. Add the scallops and any accumulated juices, the parsley, capers, lemon juice, and 1/8 teaspoon salt. Mound the puree on plates and top with the scallops and sauce.