Preheat the oven to 350 degrees.
Grab a roasting pan big enough for the brisket, potatoes, and 4 carrots, and that can also be used on the stovetop.
Warm about 4 tbsp of oil in the roasting pan over medium low heat on the stove top.
Season the brisket liberally with salt and pepper on both sides. Sprinkle 1/4 cup of flour on one side, and repeat on the other side. You can omit this, but it does help get a crust on the brisket as well as thicken the gravy as it cooks.
Place the brisket fat side down on the now hot roasting pan. Allow to cook for 5 minutes and then flip and allow the other side to cook for 5 minutes. Also, this step and be tricky because the brisket is HUGE….soooo, one thing that helps me is take two giant (like the grill ones) spatulas and place one on each side and use one to flip and the other to hold it steady.
Once browned on each side, remove from the pan.
Deglaze the pan with wine or broth. Add another tbsp or two of oil. Add the sliced onions. All of them.
Allow these guys to caramelize. This process will go low and slow. They will need about 20-25 minutes to get golden and soft. Move them around a little bit now and then. This is flavor town. Trust. While the onions caramelize, grab a food processor. Place the 2 additional carrots, the celery, and garlic cloves, a heavy pinch of salt, some cracked pepper, the tomato paste, and 3 tbsp of water in the food processor. Blend until a thick paste is made. Set aside. Once the onions are cooked down, add the brisket back into the roasting pan (fat side up).
Brush the tomato-veggie paste on top of the brisket.
Add about 1/2 cup of water to the roasting pan.
Cover the pan tightly with foil.
Place in the oven for 1 hour and 45 minutes.
Reduce heat to 325 degrees. Allow to cook for another 2- 2.5 hours.
Remove from heat, and slice against the grain.