In a large pot set over medium heat, add the canola oil and the garlic, cook for about 30 seconds. Add the onions and sauté until tender, about 4-5 minutes. Add the white wine and allow the liquid to reduce by half.
Quarter one of the lemons and add it to the pot along with the Old Bay seasoning. Add the potatoes, corn, chorizo, and just enough water to cover.
Bring the pot to simmer. Then layer the crabs, followed by the split lobsters. Cover with the lid and allow to steam for 3-4 minutes.
Add the littleneck clams, cover with the lid, and allow to steam for 3-4 minutes more. Then, add the shrimp, remove the pot from the heat, and allow to rest 5 minutes covered.
Carefully transfer the seafood and vegetables to a large serving platter, reserving the steaming liquid.
Transfer 1½ cups of the cooking liquid to a small saucepan, add the butter, and bring to a boil. Taste and adjust for seasoning, and then pour the liquid over the cooked seafood platter.
Sprinkle with the chopped parsley, cut the remaining lemon into wedges, and serve.