- 3/4 pound celery root (sometimes called celeriac), peeled with a knife and cut into 1/2-inch pieces (1 1/3 cups)
- 2 (6-inch) fresh lovage sprigs, coarsely chopped, or 1/4 cup coarsely chopped tender inner leaves from celery ribs
- 1 teaspoon table salt
- 4 cups water
- 1/2 stick (1/4 cup) unsalted butter
- 1 1/2 pound oyster mushrooms, trimmed
- 1 cup diced (1/3-inch) fennel bulb (sometimes called anise; 5 ounces)
- 1/2 cup diced (1/3-inch) sweet onion such as Vidalia or Walla Walla (2 1/2 ounces)
- 1 cup dry white wine
- 1/2 cup heavy cream
- 12 large sea scallops (1pound ), tough muscle removed from side of each if necessary
- 12 jumbo shrimp (10 to 15 per pound ), peeled
- 12 shucked Kumamoto oysters in their liquor
- 1/4 teaspoon coarsely ground black pepper
- Coarse sea salt to taste
- Garnish: fresh lovage or celery leaves, chopped
- 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. Transfer celery root with 1 cup cooking liquid (reserving remainder) to a food processor and purée until very smooth. Force purée through a fine-mesh sieve into a bowl, pressing on solids, then stir in 1 cup reserved cooking liquid.
- 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. Add wine and boil, uncovered, until reduced by half, about 3 minutes. Stir in celery purée, cream, and remaining 2 tablespoons butter. Add scallops and shrimp, then simmer, covered, turning occasionally, until shellfish are just cooked through, 5 to 6 minutes.
- Divide scallops and shrimp among 4 shallow bowls, then divide vegetables from skillet among bowls using a slotted spoon. Boil celery mixture in skillet over moderate heat, stirring occasionally, until reduced to about 2 cups, 8 to 10 minutes. 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. Spoon thickened purée over seafood and vegetables, then sprinkle with pepper and sea salt.