Bring sliced lemon, salt, and 3 cups water to a boil in a large pot fitted with a pasta strainer or steamer insert. Add crab clusters or legs; cook, covered, until shells are bright red and meat is heated through, about 8 minutes. If crab is frozen, it may take a little longer, 10–12 minutes. Uncover; transfer crab to a cutting board or paper-lined surface. Serve with dipping sauces, or with lemon halves, if you like.