Thaw leg of lamb in refrigerator for 24 to 26 hours.
Take lamb out of refrigerator and remove butcher’s twine.
Pat dry with paper towel and season generously with salt and pepper on all sides; allow lamb to come to room temperature on counter for an hour.
Preheat oven to 325 degrees Fahrenheit.
Line the bottom of a Dutch oven with orange and onion slices; set aside.
Add grapeseed oil to a large cast-iron pan and bring to medium-high heat.
Sear lamb on all sides to get a nice, browned crust – about 2 minutes on each side.
Place seared lamb in Dutch oven on top of orange and onion slices and pour Barbacoa marinade over it; shake Dutch oven to disperse marinade evenly across the bottom and add bay leaves.
Put Dutch oven in preheated oven and cook for 3 hours 30 minutes to 4 hours until fork-tender, checking for doneness at the 3-hour mark.
Remove lamb from oven and place in clean large cast-iron pan or oven-proof serving dish and shred with 2 forks, discarding any unwanted fat.
Strain the braising liquid from Dutch oven and pour desired amount of liquid over lamb; season to taste with salt and pepper.
Serve with cilantro rice, corn elote salad, and warm tortillas.