Preheat the oven to 425 degrees and spritz a large sheet pan with non-stick spray. Make the Old Bay Vinaigrette while the oven is preheating.
Add quartered potatoes and onions to one side of the prepared sheet pan and place the corn on the other. Cover all of the veggies with the butter and sprinkle with salt and pepper. Toss to coat and bake for 20 minutes while you prepare the remaining ingredientss.
Add the sausage and deveined shrimp to the section on the sheet pan with potatoes and onions and flip the corn. Drizzle all of the meat and veggies with 2 tbsp of Old Bay Viniagrette and toss to combine. Spread out in an even layer (as best as you can) and return to the oven.
Roast for an additional 10-12 minutes or until shrimp is opaque and sausage is heated through. Sprinkle with fresh parsley and toss to combine. Serve immediately as is or allow to chill in the fridge until ready to serve.
To serve, cut kernels off of corn, add it to the chilled Lowcountry Boil, and mix. Add a scoop or two of chilled Lowcountry Boil over a bed of romaine hearts with an extra drizzle of Old Bay Vinaigrette and fresh lemon wedges.
For the Old Bay Vinaigrette:
Add all of the ingredients to a mason jar, screw on the lid, and shake until it’s well mixed.
To easily peel and devein the shrimp: Pinch the legs between your thumb and forefinger and pull them down to remove. This should crack the shell making it easy to completely remove. Devein by running a paring knife along the back of the shrimp creating a small slit and exposing the black vein. Lift out the black vein with the knife, then rinse the shrimp under cold running water. Pat dry before adding salt and pepper.