Remove brisket from the fridge and allow to come to come to room temperature, about 1 1/2 hours for 8lbs. (1 hour for 4).
Preheat oven to 325°
Heat 1 tablespoon Vegetable oil in a large heavy bottomed pot or dutch oven (that has a tight-fitting lid) over medium-high heat.
Add brisket fat side down, and allow to brown undisturbed for 6-8 minutes. Turn brisket over and allow to brown on the opposite side for 5 minutes. Using tongs transfer to a serving platter.
Reduce heat to medium, add remaining 1 tablespoon of vegetable oil.
Add onion, celery, carrots, bay leaves, and thyme sprigs, cook stirring frequently for 10 minutes to allow the flavors of the aromatics to begin to release and the tough exteriors to soften.
Toss in the crushed garlic and chopped plum tomatoes, cook for an additional 2 minutes.
Add the stock, stout, bourbon, light brown sugar, and soy sauce. Stir and bring to a simmer.
Place Brisket, fat side up back into the pot.
Cover the top of the pot with heavy-duty aluminum foil and then with a tight fitting lid.
Transfer to the preheated oven and cook for about 4-5 hours with an internal temperature reading of 165°-175°F/74°-79°C. Brisket should be tender and still maintain its shape.
Remove for the pot with a large spatula, set aside on a roasting pan fat side up.
Strain the braising liquid into a large bowl. Return the liquid to the pot and bring to a simmer, until reduced to about 3 cups (about 15-20 minutes). Preheat the broiler.