Preheat your oven to 425°F.
Lightly grease a large rimmed baking sheet or line it with parchment paper for easier cleanup.
Add the potatoes, corn, and sliced onion to a large bowl. Cut the potatoes into roughly 1-inch pieces so they cook evenly and are tender by the time the shrimp are finished cooking.
In a separate bowl, combine the peeled and deveined shrimp with the sliced andouille sausage. Set aside.
Melt the salted butter.
Stir in the minced garlic and Cajun seasoning until well combined.
Pour half of the Cajun garlic butter over the potatoes, corn, and onion.
Toss until everything is evenly coated.
Spread the potatoes, corn, and onion in an even layer on the prepared baking sheet.
Bake at 425°F for 18 minutes.
While the vegetables are roasting, pour the remaining Cajun garlic butter over the shrimp and sausage.
Toss until evenly coated.
After 18 minutes, carefully remove the sheet pan from the oven.
Add the seasoned shrimp and andouille sausage to the pan, distributing them evenly among the vegetables.
Return the sheet pan to the oven and bake for an additional 8–10 minutes, or until the shrimp are opaque and cooked through and the potatoes are fork-tender.
Remove from the oven and sprinkle with freshly chopped parsley.
Serve immediately with fresh lemon wedges for squeezing over the top.