Bring a large pot of water to a boil and season generously with Old Bay and Cajun seasoning.
Add potatoes and cook for 10 minutes, then add corn and sausage. Cook for another 5 minutes.
Add shrimp and crab legs, cooking for 5 more minutes until seafood is cooked through.
In a separate pan, melt butter and sauté garlic until fragrant.
Remove seafood and vegetables from the pot, toss with garlic butter sauce, and garnish with fresh parsley.
Serve hot with lemon slices on the side.