Preheat the oven to 450 degrees F. Line a large rimmed baking sheet with parchment paper. Place the potatoes on the baking sheet and drizzle with oil. Salt and pepper, then roast in the oven or 20 minutes.
Meanwhile, cut the sausage into 1/2-inch slices. Slice the onion into 1/2-inch wedges. Cut each ear of corn into 4 pieces.
After the potatoes have roasted, add the sausage, onions, and corn to the sheet pan. Sprinkle with Old Bay seasoning and place back in the oven for 10 minutes.
Move the sausage and veggies to the sides of the sheet pan. Spread the shrimp across the middle of the pan in a single layer. Sprinkle with Old Bay Seasoning. Then lay the crab legs across the shrimp. Place back in the oven for approximately 5 minutes, or until the shrimp are pink.
Remove from the oven. Drizzle the entire pan with the juice of half a lemon and melted butter. Toss to coat. Then garnish with parsley and extra lemon.
NOTES: All brand names listed in recipe come from ALDI.