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1
Simmer celery root, lovage, and 1/2 teaspoon table salt in water in a 3-quart saucepan, partially covered, until tender, 12 to 15 minutes.
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2
Transfer celery root with 1 cup cooking liquid (reserving remainder) to a food processor and puree until very smooth.
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3
Force puree through a fine-mesh sieve into a bowl, pressing on solids, then stir in 1 cup reserved cooking liquid.
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4
Heat 2 tablespoons butter in a 2- to 3-inch-deep 12-inch heavy skillet over moderate heat until foam subsides, then cook mushrooms, fennel, onion, and remaining 1/2 teaspoon table salt, covered, stirring occasionally, until softened, 10 to 12 minutes.
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5
Add wine and boil, uncovered, until reduced by half, about 3 minutes.
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6
Stir in celery puree, cream, and remaining 2 tablespoons butter.
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7
Add scallops and shrimp, then simmer, covered, turning occasionally, until shellfish are just cooked through, 5 to 6 minutes.
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8
Divide scallops and shrimp among 4 shallow bowls, then divide vegetables from skillet among bowls using a slotted spoon.
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9
Boil celery mixture in skillet over moderate heat, stirring occasionally, until reduced to about 2 cups, 8 to 10 minutes.
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10
Add oysters, including their liquor, and cook over moderately low heat until edges begin to curl, about 1 minute, then transfer to bowls with slotted spoon.
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11
Spoon thickened puree over seafood and vegetables, then sprinkle with pepper and sea salt.
