Heat brisket in a large skillet or dutch oven over medium-high heat. Cook until browned on all sides. Stir in water, ketchup, vinegar, onions, garlic, brown sugar, and salt. Bring to a boil, then cover and reduce heat to medium-low. Continue simmering until tender, turning brisket occasionally, 2 hours and 30 minutes to 3 hours and 30 minutes.
Remove brisket and allow to cool before slicing the meat against the grain. Place brisket slices in a 9x13 inch baking pan or large platter and pour gravy on top. Cover and refrigerate overnight. Remove any excess fat and reheat before serving.