4 pounds medium red potatoes, scrubbed but not peeled
1 cup mayonnaise
1 sweet onion, finely chopped
1/4 cup plus 2 tablespoons fresh lemon juice
3 tablespoons Dijon mustard
Bring a large saucepan of water to a boil. Add the lobster claws, cover and bring back to a boil. Uncover and boil the claws over moderate heat for 6 minutes. Drain and let cool, then refrigerate until cold. Crack the claws and remove the meat in one piece.
In a large pot, cover the potatoes with water and bring to a boil. Add salt and simmer over high heat until tender, about 25 minutes. Drain the potatoes and let cool, then halve them.
In a medium bowl, combine the mayonnaise, onion, lemon juice and mustard; season with 1 1/2 teaspoons of salt and the pepper. Fold in the lobster and potatoes and refrigerate until chilled or for up to 3 hours. Serve cold.