
Gather and prepare all ingredients.
In a large stockpot, bring 3 quarts of water to a rolling boil with Old Bay seasoning, red onion, garlic, and one quartered lemon.
Add the potatoes and salt. Cook 8–10 minutes until just fork-tender.
Add the corn and sausage. Cook for 3 minutes.
Add the shrimp and cook for 2 minutes, until they are pink and opaque.***
Drain the water and transfer the shrimp mixture to a large platter or sheet pan.
Heat the butter in a small bowl in the microwave for 30 seconds to 1 minute, or until melted.**** Stir in Cajun seasoning and parsley.
Serve immediately with lemon wedges and the seasoned butter for dipping.