If crab legs are frozen, thaw them in the refrigerator overnight or run under cold water for 10-15 minutes until pliable.
Fill a large stockpot with about 2 inches of water. Add the lemon slices and Old Bay seasoning to the water. Place a steamer basket inside, ensuring the water does not touch the bottom of the basket.
Bring the water to a rolling boil over high heat.
Arrange the crab legs in the steamer basket. Cover the pot tightly with a lid.
Steam for 5 to 7 minutes if the crab is thawed, or 8 to 10 minutes if frozen, until the meat is heated through and emits a cooked seafood aroma.
While the crab steams, prepare the dipping sauce. In a small saucepan, combine the melted butter, minced garlic, and a squeeze of fresh lemon juice. Heat gently for 1-2 minutes to infuse flavors, then stir in the parsley.
Remove the crab legs from the pot with tongs. Serve immediately with the warm garlic butter sauce.