Add the onion, garlic, cilantro, mint, jalapenos and fresh oregano (if using) to your food processor and pulse until very finely chopped, scraping the sides down as needed. Transfer to a large bowl and stir in all remaining mojo marinade ingredients.
Remove 1 1/2 cups of the marinade and store in an airtight container in the refrigerator (to serve with roasted pork).
Transfer the rest of the marinade to a freezer bag and add the pork; turn to coat, squeeze out excess air and seal. Marinate the pork in the refrigerator for 12-24 hours (no longer!), turning the bag occasionally.
When ready to cook, remove the pork from the refrigerator and let rest on the counter for 1 hour.
Preheat the oven to 425 degrees F. Heat vegetable oil over medium-high heat in a large cast iron skillet. Wipe excess marinade off pork, then sear on all sides until browned.
Place the pork on a rack in a roasting dish (or use halved onions to prop up if you don’t have one) with 1 cup of water in the bottom. Spoon some of the marinade from the bag over the top of the pork. Cover the pork with foil, slightly tenting in the middle so it is not touching the pork.
Bake at 425 F for 30 minutes then reduce the temperature to 325 F and continue to roast for an additional 3 hours, adding additional water as needed so the water doesn’t dry up. Only check the water once an hour but don't stress it, it’s not the end of the world if you need to leave and it dries up.
After 3 hours, remove the foil and baste the pork with the water/drippings in the bottom of the pan. Roast for an additional 30 minutes. The meat should very tender; about 180-190 F on an instant read thermometer, without touching the bone. (Although this pork does not require a meat thermometer because it will be cooked beyond overdone until tender.) If it's not tender, cook for an additional 30 minutes as needed.
At this point, you can turn your oven to broil (without moving the pork) for a more caramelized exterior if desired; broil for 5-10 minutes, keeping an eye on it so it doesn’t burn.
Transfer the pork to a cutting board and loosely tent with foil. Rest for 20 minutes before carving.
Meanwhile, transfer the 1 1/2 cups reserved UNUSED mojo sauce to a saucepan and simmer for 5 minutes while stirring occasionally, or until reduced by half. Stir in a couple tablespoons of drippings if desired for more succulent flavor. Slice the pork and brush with some of the mojo. Serve remaining mojo on the side.