1. Heat two tablespoons of oil in a large skillet over medium-high heat; add onions and garlic. Cook until brown and tender, 10 to 15 minutes. Season with salt and pepper.
2. Remove the meat from marinade and add to skillet. Lower heat, cover, and cook for two hours. During the last 15 minutes, uncover and add remaining one-fourth cup barbecue sauce and cornstarch mixture. Cook until sauce thickens.
If you like your sauce thick, use two tablespoons cornstarch. If you’ll be reheating the sauce, it will thicken on its own and one tablespoon will suffice. Use the extra sauce for dipping.