The sweetness of the lobster and tomatoes is balanced by a light but flavorful lemon-orange dressing. Tarragon adds a fresh zing.
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 lobsters to ice-water bath using tongs. Let stand for 10 minutes to cool.
Meanwhile, whisk together oil, zests and juices, mustard, shallot, and 1/2 teaspoon salt.
Crack lobster claws, knuckles, and tails, and remove meat. Slice tail meat 1/2 inch thick, and leave claws and knuckles whole.
Combine lobster, greens, tomato, and tarragon. Drizzle with dressing, and toss to coat. Garnish with tarragon. Serve immediately.
Dressing can be refrigerated for up to 1 day; lobster meat can be refrigerated overnight.