1. Heat two tablespoons oil in a frying pan over medium heat. Sauté onion until soft, then set aside in a medium-sized bowl.
2. Meanwhile, preheat the slow cooker to high. Add one tablespoon of oil.
3. Use the same frying pan to sear the meat for about three minutes on each side. Transfer to slow cooker with all the pan juices.
4. Add sauce ingredients to the bowl with the onions and mix until combined.
5. Pour sauce over the meat, coating it as best you can.
6. Cover and cook on high for three to four hours, then reduce to low for up to eight hours.