Sprinkle brisket with seasoned salt and black pepper.
Heat oil in a Dutch oven over medium-high heat. Sear brisket in hot oil until browned on both sides. Transfer brisket to a large plate. Cook and stir onions and garlic in the Dutch oven until vegetables are softened, about 6 minutes. Return brisket to the Dutch oven.
Whisk beef broth, ketchup, brown sugar, vinegar, chili powder, bay leaves, and cayenne pepper together in a bowl; drizzle broth mixture over brisket.
Bring beef mixture to a boil; reduce heat to low and cover. Simmer until beef is very tender and easily falls apart when pulled with a fork, 2 1/2 to 3 hours. Remove and discard bay leaves. Transfer beef to a platter, reserving liquid in the Dutch oven. Slice beef.
Whisk water and flour together in a small bowl; stir into the Dutch oven and bring to a boil until sauce is thickened. Serve sauce over sliced brisket.