Sweat turnip, celery root, and potato in butter without coloring. Add reserved clam stock, cream, oyster liquor, thyme, salt, and pepper, and simmer until vegetables are tender. Add oysters, razor clams, and manila clams. Cook for 30 seconds. Season. Spoon into serving bowls. Drizzle with truffle oil and top with truffle shavings.;
Melt butter in a pot. Add onion, leek, and fennel and sweat gently. Add clams and white wine. Reduce wine by half. Add water and aromatics. Simmer gently for 15 minutes. Strain through a fine chinoise and reserve. Shuck clams and reserve.