Step 1

Melt the butter over medium heat in a large (8- to 10-inch) saucepan or Dutch oven. Rinse the leeks well in a colander, spin them dry in a salad spinner, and add them to the pot. Sauté them over medium-low heat, stirring occasionally, for 8 to 10 minutes, until tender but not browned. Add the potatoes, celery root, cream, 4 teaspoons salt, and 2 teaspoons pepper to the pot, stir, and bring to a boil. Lower the heat to very low, cover the pot, and simmer gently for 25 to 30 minutes, stirring occasionally, until the vegetables are tender. Be careful—don’t let the vegetables scorch on the bottom of the pan! In batches, pour the mixture into the bowl of a food processor fitted with the steel blade and pulse until coarsely puréed. Taste for seasonings, return to the saucepan, and keep warm over very low heat. If the mixture gets too thick, add a little more cream.

Step 2

If it hasn’t already been removed, peel off the tough strip of muscle on the side of each scallop. Pat the scallops dry with paper towels and season them generously on both sides with salt and pepper. Heat two 12-inch sauté pans over medium-high heat. Add 2 tablespoons of grapeseed oil to each pan and, when the oil is almost smoking hot, add half the scallops to each pan. Cook undisturbed for about 3 minutes, until golden brown on the bottoms. Don’t crowd the scallops or they’ll steam rather than sear. Using a small metal spatula, turn the scallops and cook for 2 to 3 minutes more, until just cooked through. (If the pan is hot enough and you let the scallops cook undisturbed, they won’t stick to the pan.)

Step 3

Spoon the warm purée onto 6 dinner plates. Place the scallops on the purée, drizzle with the basil oil, and sprinkle with the chives. Serve warm.