Preheat the oven to 400 degrees F.
In a saute pan over medium-high heat, add the oil and heat to the verge of smoking. Add the pancetta, reduce the heat to medium, and cook the pancetta until crispy, 2 to 3 minutes. Once cooked, remove the pancetta with a slotted spoon to paper towels and reserve. Add the onions and garlic and cook until translucent, 6 to 7 minutes. Once the onions are cooked, remove from the pan and toss in a bowl with the lemon juice, crab boil seasoning, butter, and parsley. Top the clams with the onion-herb mixture, reserved pancetta, and Parmesan. Place on a baking sheet and bake until the cheese has melted, 10 to 12 minutes.