Heat a large Dutch oven or pot over medium-high heat and add oil. Season brisket liberally with salt and brown brisket until all sides are browned, about 5-7 minutes per side.
Remove the brisket, lower heat to medium and add in the onion. Saute for 5 minutes until they start to brown and soften, adding in more oil if needed. Then add in the garlic and saute for 1 minute. Then in water, white vinegar, ketchup, onions, garlic, and sugar. Stir to scrape up all the good bits stuck to the pan.
Add the brisket back in. Bring to a boil, and then cover and lower to a simmer over low hea.
Cook brisket until the sauce has thickened and the brisket is fork tender or about 3-4 hours.
Once brisket cools, slice against the grain or you can also refrigerate and slice the next day before reheating. Serve with sauce.