Season the brisket with seasoned salt and black pepper.
Heat oil in a Dutch oven over medium-high heat. Brown the brisket on both sides and transfer it to a large plate.
Cook onions and garlic in the Dutch oven until softened, about 6 minutes. Return the brisket to the Dutch oven.
In a bowl, whisk together beef broth, ketchup, brown sugar, vinegar, chili powder, bay leaves, and cayenne pepper. Drizzle this mixture over the brisket.
Bring the mixture to a boil, then reduce the heat to low, cover, and simmer for 2 1/2 to 3 hours, or until the beef is tender and easily falls apart when pulled with a fork. Remove and discard the bay leaves. Transfer the beef to a platter and slice it.
In a small bowl, whisk together water and flour. Stir this mixture into the liquid in the Dutch oven and bring it to a boil until the sauce thickens.