Score the pork leg. Add it to a large plastic zip-top bag with 14 ounces of the mojo, sealing it and making sure the pork is coated thoroughly. Place in refrigerator overnight.
Pour the pork leg and marinade into the crock of your slow cooker. Cook on High until the thickest part of the leg registers 195-200 degrees Fahrenheit on an instant-read thermometer. Using two forks, shred the meat into bite-sized pieces. If not ready to finish the dish, set the slow cooker to warm and let the pork stay in the crock.
Move a rack to the top cooking position in your oven, then preheat the broiler. Turn a burner on your stovetop on to medium-high heat. Place the vegetable oil into a skillet on that burner. Once the oil is shimmering, add the onions and cook, moving them around occasionally, until the onions are softened and browning slightly, approximately 5-7 minutes.
Line a rimmed baking sheet with aluminum foil. Mix the shredded pork leg from the slow cooker with the onions from the skillet, then scatter them on the lined baking sheet. Place in oven for 2-3 minutes until caramelization occurs, then pull the baking sheet out and flip the pork and onion mixture, making sure to get fresh pork and onions to the top of the sheet. Repeat this process 2-3 times, until most of the pork and onions are caramelized.
Pour the remaining 10 ounces of mojo from the plastic container into a small saucepan. Add an equivalent amount of drippings from the slow cooker. Turn a burner on the stove to medium-high heat and cook, whisking frequently at first, until the sauce begins to boil. Lower the heat to medium-low and let sauce simmer until reduced slightly, whisking occasionally.