Put the roast and mojo criollo sauce in a large plastic bag. If the roast is very thick, you may cut it in half before marinating.
Refrigerate 24-48 hours, turning 2-3 times daily.
Heat oil in a large skillet and lightly brown the roast, 2-3 minutes on each side. Place roast in a slow cooker.
In a mixing bowl, wisk together the reserved marinade, water, pork gravy mix and vinegar. Pour over the roast. Add bay leaves and sliced onions.
Cook on LOW for 5-6 hours. (Slow cookers may vary.).
Remove the pork and onions from the cooker, reserving the gravy. Discard the bay leaves.
Served sliced or shredded. SLICED: Slice the pork into medallions and serve with gravy and onions. SHREDDED: Shred the pork, adding some gravy to keep it moist. Pile onto crusty Cuban bread and serve with gravy for dipping.