This is a very popular dish in Florida, where Cuban and Spanish influence on the food abounds. The sour orange juice is really unique and the key to this dish. If it's not available to you locally, look around on the Internet. It may seem like you use a lot of salt here, but that is also a key to the authentic flavor. It's a simple prep--just make sure you get it marinating a day ahead of cooking. This dish should always be served with black beans and rice.
Put the garlic in the bowl of a food processor fitted with a metal blade. In a medium bowl, mix together the orange juice, salt, olive oil, cumin, oregano, and pepper. Pour the mixture over the garlic. Process on high for about 20 seconds, until the mixture is well blended. Transfer the paste mixture to a small bowl. Rub the paste mixture all over the leg, shoving it deeply into all of the holes. Put the leg in a large plastic bag and toss in the onion, spreading it evenly around the leg. Seal the bag and refrigerate for at least 12 hours and preferably 24.
Prepare your cooker to cook indirect at 235 degrees F using mild pecan wood for smoke flavor. Take the pork leg out of the bag and place it, fat-side up, in an aluminum foil pan. Place the pan in the cooker for 2 hours. Flip the leg over and cook for 2 hours more.
Meanwhile, make the mojo: Heat the olive oil in a small saucepan over medium heat. Add the garlic and cook, stirring often, for 2 minutes. Add the salt, pepper, cumin, and oregano and cook for 3 minutes, stirring often. Add the orange juice and stir well. Remove from the heat and set aside.
Flip the leg again and baste with half of the mojo. Cook for 30 minutes. Flip the leg and baste with the remaining mojo. Cook for 30 minutes. Flip the leg again and cover the pan with aluminum foil. Continue cooking until the internal temperature of the meat reaches 195 degrees F, about 3 to 4 hours more.
Remove the pan and transfer the leg to a platter. Tent it loosely with foil and let it rest for 15 minutes. Transfer the liquid from the pan to a bowl or large pitcher. Let it rest, then skim off as much grease as possible. Pull the roast off of the bones in chunks and transfer them to a platter, discarding all the bones and fat. Serve with the mojo on the side as a sauce.