This lighter take on the lobster roll exchanges the mayo for a lemon-avocado puree and the usual buttered roll for a slice of grilled bread.
Prepare an ice-water bath. Bring a large pot of water to a boil. Reduce heat to medium-low. Plunge lobsters into water headfirst, and simmer, covered, for 11 minutes. (Do not let water boil.) Transfer to ice-water bath using tongs. Let stand for 10 minutes to cool.
Crack lobster claws, knuckles, and tails, and remove meat. Chop meat into chunks.
Toast fennel seeds in a skillet over medium-high heat for 1 minute; grind. Transfer to a food processor; pulse with avocado, 2 tablespoons lemon juice, 2 teaspoons scallions, 1/2 teaspoon salt, and some pepper until smooth.
Spread avocado mixture onto bread; top with watercress. Arrange lobster meat on top, and sprinkle with remaining scallions. Drizzle with remaining tablespoon lemon juice and the oil. Garnish with lemon zest.