Preheat the oven to 275 degrees F.
Place the brisket in an oven-safe pan. Combine the ketchup and soup mix in a bowl, and then stir it around to mix. Pour in 1 cup water and stir to combine. (You can add more water if needed!)
Pour the sauce all over the brisket, and then flip the brisket over to coat the other side. Now just cover the pan with foil and roast in the oven for 6 to 7 hours.
Remove the pan from the oven and test the brisket to make sure it's fork-tender. The brisket should fall apart if you look at it! Slice the brisket into thin strips, then return the beef to the sauce and keep it warm until you're ready to serve.