Step 1: Prepare Your Ingredients. Thoroughly wash all your seafood. Peel and devein shrimp (leave tails on for presentation if desired). If using whole crabs, clean them according to your fishmonger's instructions. Chop your potatoes into halves or quarters if large, and break corn on the cob into 2-3 inch pieces. Slice lemons and onions, and mince garlic.
Step 2: Assemble the Boil Liquid. In your largest stockpot (at least 12-quart), combine water (or a mix of water and seafood broth), the generous amount of Cajun seasoning, bay leaves, half of your sliced lemons, sliced onions, and half of the minced garlic. Bring this mixture to a rolling boil over high heat.
Step 3: Add Potatoes and Sausage (if using). Once the liquid is boiling, add your chopped potatoes and any smoked sausage. Reduce heat slightly and let them cook for about 10-15 minutes, or until the potatoes are just tender-crisp.
Step 4: Introduce the Corn. Add the corn on the cob pieces to the pot. Continue to boil for another 5-7 minutes, until the corn is vibrant yellow and tender.
Step 5: Add Harder Shellfish. Now it's time for the crab legs or other harder-shelled seafood. Add them to the pot and cook for 5-8 minutes, depending on size, until they are bright red and cooked through.
Step 6: Add Shrimp and Softer Shellfish. Finally, add your shrimp, mussels, and clams. Cook for only 2-4 minutes, or until the shrimp turn pink and opaque, and mussels/clams open. Be careful not to overcook the shrimp, as they can become rubbery.
Step 7: Prepare the Garlic Butter Sauce. While the seafood is cooking, melt unsalted butter in a separate saucepan over medium-low heat. Add the remaining minced garlic, a pinch of Cajun seasoning, and a squeeze of fresh lemon juice from the remaining lemon halves. Simmer gently for 2-3 minutes to allow the flavors to meld.
Step 8: Serve Your Feast. Carefully drain the seafood and vegetables from the boiling liquid. Transfer everything directly onto a large table covered with newspaper or butcher paper. Drizzle generously with the warm garlic butter sauce. Garnish with fresh parsley and extra lemon wedges. Dig in and enjoy your incredible seafood boil!