Chop the medium yellow onion and smash the garlic cloves. In your slow cooker, combine the onion, garlic, and bay leaves, creating an aromatic base. Set your slow cooker to low heat.

Wash and halve the baby potatoes, then layer them over the aromatics in the slow cooker. Cut the ears of corn into manageable pieces, and scatter them over the potatoes.

In a separate bowl, whisk together the low-sodium seafood broth, white vinegar, mustard seeds, celery salt, and Old Bay seasoning. Pour this mixture over the layered vegetables in the slow cooker.

Slice the andouille sausage and layer the pieces on top of the broth and vegetables. Add the large peeled shrimp, ensuring they remain on top without stirring.

Cover the slow cooker and cook on low for about 4 hours, until the shrimp are opaque and the baby potatoes are tender.

Stir in the room temperature unsalted butter until melted. Sprinkle chopped fresh parsley over the dish and serve warm with lemon wedges.