Heat the olive oil in a large heavy dutch-oven ( I used a 5.5qt cast-iron Le Creuset) over high heat. Season the brisket with salt and pepper. Add the brisket to the dutch-oven and sear it on both sides until brown, about 5 minutes per side. Transfer the brisket to a large plate.
Add the sliced onions to the pot. Stir and sauté until they are softened and golden brown, about 8 minutes. Place the brisket on top of the onions. Add the sauce. Bring to a boil. Cover the pot and transfer to the preheated oven.
Bake for 3 hours. Let the brisket cool for 30 minutes before slicing. Transfer the brisket to a cutting board and thinly slice across the grain.
Spoon any fat from the sauce.
To serve: place meat slices on a large platter and spoon the sauce over the meat.