1. Preheat oven to 400°F or preheat grill to medium-high heat.
2. Boil potatoes until nearly tender, about 10 minutes, then add corn and boil for an additional 3–5 minutes. Drain and set aside.
3. In a large bowl, mix shrimp, sausage, corn, potatoes, and optional sliced onion.
4. In a separate bowl, combine melted butter, Old Bay (or Cajun) seasoning, minced garlic, lemon juice, salt, and pepper. Pour half the mixture over the ingredients and toss gently.
5. Divide mixture onto large, heavy-duty sheets of aluminum foil (~12×12 inches). Fold foil edges to create sealed packets.
6. Cook packets on grill for 8–10 minutes per side or bake in oven for 15–20 minutes, until shrimp are pink and cooked through.
7. While packets cook, melt remaining butter in a saucepan, stirring until it browns lightly.
8. Serve packets topped with fresh parsley, lemon wedges, and the browned butter.