Pat the brisket dry and season it with salt and pepper to taste.
Heat a skillet over medium-high heat, slick it with just a little oil, then add the brisket. Sear it well, turning until all sides are well-browned.
Place the seared brisket in a heavy ovenproof pot or pan such as a Dutch oven.
Wipe the skillet clean and then add the butter or oil and onions and return to medium-highish heat. Cook, stirring occasionally, until pretty well browned, 7 to 10 minutes.
Sprinkle the brisket with pepper and then add the browned onions, allspice, and bay. Cover tightly and transfer to the oven for about 10 to 15 minutes. Reduce the heat to 225°F (107°C) and let cook, undisturbed, for 2 1/2 hours.
Transfer the brisket to a cutting board to rest. Reserve the pan juices in the skillet.
Spoon off any visible fat from surface of the juices and discard the bay leaf. Spoon the onions into a sieve or strainer, place it over the pan juices, and press the onions through the sieve, discarding any solids. If desired, strain again and reduce over medium heat for a few minutes. Taste and season the pan juices with salt and pepper to taste.