Fill a large stockpot with handled strainer with 4 quarts water (if using 2 pots, divide ingredients and water between them).
Add lemon, salt and peppercorns and Zatarains. Cover and bring to a boil.
Add potatoes and return to a boil; cook for 7 minutes.
Add Andouille sausage (or sausage of your choice) and onion; cook 5 minutes.
Add corn, return to a boil and cook for another 5 minutes.
Add crab and shrimp, which will be on top and should be more steamed and not soaked in water. Cover and cook an additional 5 minutes until shrimp is cooked through.
Remove the strainer from pot and let crab boil drain, then transfer to a large bowl or toss on a table.
Serve with your choice of condiments.