Preheat oven to 300 degrees.
Combine water, ketchup, vinegar, onions, garlic, brown sugar and Kosher salt in a large mixing bowl.
Place the brisket in a large baking dish, then pour the ketchup mixture over the brisket.
Cover the baking dish tightly with aluminum foil. Bake for 4 hours until meat is fork-tender.
Serve thinly sliced, against the grain.
