Preheat the oven to 275 F.

Using a sharp knife, remove the fat as much as you can from the brisket. Add it to an extra large roasting pan (I often use large aluminum tins).

In a large bowl, add the onion soup, ketchup, mustard, jelly, brown sugar, and water. Mix well to combine.

Pour mixture over the brisket, making sure the entire brisket is covered well. Cover the roasting pan with aluminum foil and bake in the preheated oven for 6-8 hours. A good rule of thumb is about 1 hour per pound of brisket to cook. The brisket should be tender enough that it cuts very easily with a knife or can even be firmly flaked apart with a fork.

Cut off any additional fat that wasn’t cut off before cooking. To cut the brisket: let it cool and then slice thinly against the grain. Pour the extra liquid into a bowl. Pour some on top of the brisket before serving and serve the remaining liquid on the side for guests to pour themselves.