01 -
Preheat the oven to 275°F. Place the brisket fat side up in a roasting pan that fits it snugly.
02 -
Stir the ketchup, dry onion soup mix, and water in a bowl until the soup mix is evenly moistened.
03 -
Pour the sauce over the brisket. Turn the brisket once so both sides are coated, then leave it fat side up.
04 -
Cover the pan tightly with foil. Bake for 5 1/2 to 6 1/2 hours, until a fork slides into the thickest part with little resistance.
05 -
Remove the pan from the oven and let the brisket rest, still covered loosely, for at least 30 minutes.
06 -
Move the brisket to a cutting board. Slice across the grain into 1/2-inch slices and serve with the warm pan sauce.