Step 1: In a very large stockpot (at least 12-quart capacity), combine water or broth, quartered onion, halved garlic head, sliced lemons, Old Bay seasoning, Cajun seasoning, smoked paprika, garlic powder, onion powder, and cayenne pepper. Bring the mixture to a rolling boil over high heat.
Step 2: Add the red potatoes to the boiling liquid. Reduce heat slightly and cook for 10-12 minutes, or until the potatoes are just tender when pierced with a fork.
Step 3: Add the corn on the cob and Andouille sausage to the pot. Continue to cook for another 5 minutes.
Step 4: Stir in the melted butter. This creates the rich, flavorful base for the Seafood Boil sauce.
Step 5: Add the crab clusters to the pot. Cover and cook for 5-7 minutes, allowing them to steam and heat through.
Step 6: Finally, add the shrimp and mussels. Cover the pot and cook for an additional 3-5 minutes, or until the shrimp are pink and opaque and the mussels have opened. Discard any unopened mussels.
Step 7: Carefully drain the cooking liquid, or use a large slotted spoon to transfer all the delicious contents to a very large serving platter or directly onto a newspaper-lined table. Garnish with fresh chopped parsley and serve immediately with extra lemon wedges.