Butcher the full packer brisket. Split the deckle from the flat and remove excess fat. Cube the flat into 1-inch (2.5cm) cubes and place in a bowl. Cube the deckle into 1/2-inch (1cm) cubes and place in another bowl. Retain some of the fat.
Distribute the salt and pepper across the two bowls of diced meat, then stir so all the meat is evenly covered.
Preheat the large 12-inch Dutch oven and add some of the fat, cooking until rendered down and there is a layer of fat to fry in.
Add the diced brisket flat and brown on all sides, in batches if necessary.
Add water to the Dutch oven until the brisket is covered. Place on heat to allow the brisket to gently simmer.
In the 10-inch Dutch oven add some fat and allow to render down. When there is a layer of fat add the brisket deckel and brown on all sides.
Add the onions to the deckle and cook until it starts to brown. Add water until the meat is just covered and leave on the heat to a gentle simmer.
Continue to adjust the Dutch ovens over the heat making sure to maintain a gentle simmer, occasionally stirring and adding water if needed.
The brisket flat is ready when tender yet still holding together. At this point leave the lid off to let the liquid reduce down. The brisket deckle is ready when the meat is soft and falling apart when stirred vigorously. As it falls apart it will create a rich thick sauce.
Combine the dekle brisket sauce into the larger Dutch oven containing the brisket flat to create the complete salt and pepper brisket.