Season the brisket all over with salt and pepper. Heat the olive oil in a large skillet over medium-high heat, then sear the brisket on each side for 4-5 minutes until browned. Transfer the brisket to a plate.
Add the onions, carrots, celery, and garlic to the skillet and cook until the vegetables are tender, about 6-7 minutes.
Pour in the wine to deglaze the pan, scraping up any brown bits.
Add the tomato sauce, prunes, thyme, fish sauce, and bay leaves to the pan and stir until combined. Bring to a low simmer, then remove the pan from the heat.
Pour the sauce into the bottom of a large roasting pan, then nestle in the brisket, fat side up. Tightly cover the pan with foil and transfer it to the oven.
Braise the brisket for about 4 hours, or until it is completely fork tender. Transfer the brisket to a cutting board and allow to rest for 15 minutes.
Remove the bay leaves from the roasting pan and skim any fat off the surface of the sauce. Season with salt and pepper, to taste.
Thinly slice the brisket against the grain and arrange it on a large platter. Spoon the sauce on top before serving.