How to Make It
In a pot of boiling salted water, cook the lobsters over high heat until bright red all over, about 12 minutes. Drain and let cool slightly. Twist off the tails and claws. Crack the claws and remove the meat. Using kitchen scissors, slit the tail shells lengthwise and remove the meat. Discard the vein that runs the length of the tail. Cut the lobster into 1-inch pieces.
In a large skillet, cook the bacon over moderate heat until crisp, about 5 minutes. Drain well on paper towels, then coarsely chop the bacon.
In a small saucepan of boiling water, cook the green beans until just tender, about 3 minutes. Drain and refresh under cold running water. Pat dry with paper towels and cut into 1/2-inch pieces.
In a medium bowl, whisk together the buttermilk, sour cream, horseradish, mayonnaise, lemon juice and mustard. Season with salt and pepper.
In a large bowl, toss the iceberg lettuce with 1/4 cup of the dressing and transfer to a shallow glass bowl. Add the lobster, bacon, eggs, green beans, tomatoes, carrots, onion and cucumber to the large bowl and toss with the remaining dressing. Mound the salad on top of the lettuce and serve at once.