This robust seafood stew will feed a crowd.
Bring a stockpot of water to a boil over high heat. Add lobsters, cover, and cook 3 minutes; drain. Hold lobsters with a towel and twist off tails and claws. Remove meat from tails and claws; refrigerate. Reserve lobster shells and bodies.
Cut 1 leek into 1/4 inch dice; let stand in a bowl of water for 5 minutes. Lift out of water; drain. Chop 1 carrot, celery, half the fennel, and 1 clove garlic into 1/4 inch dice.
Heat 1 teaspoon butter in a large saucepan over medium heat. Add the chopped leek, onion, carrot, celery, fennel, garlic, fennel seeds, 6 sprigs thyme, and tarragon; cook, stirring occasionally, until vegetables are soft and aromatic, 3 to 4 minutes. Add lobster shells and bodies; cook, stirring occasionally, 2 to 3 minutes.
Add wine, bay leaf, tomatoes, parsley, and 8 cups water. Raise heat to high; bring to a boil. Reduce heat to medium low; simmer until very flavorful, 1 hour.
Meanwhile, halve remaining leek and cut into 1/4-inch-thick slices. Place in a bowl of water for 5 minutes; lift out of water and drain. Slice remaining carrots into 1/8-inch-thick matchsticks. Cut remaining fennel into 1/4-inch dice. Using the side of a knife, smash remaining garlic clove; set vegetables aside.
Remove stock from heat and strain, pressing on solids. Discard solids.
Melt remaining 2 teaspoons butter in a large shallow saucepan over medium heat. Add sliced leeks, carrots, fennel, garlic, remaining 2 sprigs of thyme, salt, and pepper; cook until tender, about 7 minutes. Add stock; raise heat and bring to a boil. Reduce heat to medium low; simmer for 20 minutes. Meanwhile, slice lobster-tail meat into medallions but leave claw meat whole; set aside. Add scallops and halibut to stock; adjust heat to maintain bare simmer and poach for about 1 minute. Add lobster meat, adjust heat again, and poach until fish is opaque and lobster is cooked through, 3 to 4 minutes. Discard garlic clove and thyme sprigs. Stir in watercress and sauternes, if using. Serve immediately.
One-pound lobsters (chicks) and lobsters with only one claw (culls) are the least expensive ones; use either kind in this seafood stew.