King-crab legs are sold steamed and flash-frozen. Thaw them in the refrigerator for at least eight hours and up to one day before serving.
Bring butter to a simmer in a small saucepan over medium-low, skimming foam from surface. Remove from heat; let stand until milk solids sink to bottom. Slowly pour clarified butter into a small bowl, leaving milk solids behind; discard solids. Stir in miso and lemon juice. Serve crab legs, with warm butter mixture alongside.