Preheat the oven to 325 degrees F. Pour the oil into a large skillet and set over medium-high heat. Season the beef with salt and pepper, sear 2 to 3 minutes on each side, and then set in a single layer in a casserole. Drain all but 1 Tbsp. of the oil and rendered beef fat from the skillet. Add the onions and cook, stirring, 3 to 4 minutes. Add the barbecue sauce and stock to the skillet and bring to a simmer. The moment it comes to a simmer, pour it over the beef. Cover and cook the beef in the oven for 90 minutes, or until very tender.