Trim the fat off the brisket if it is not already trimmed.
Place the brisket in a roasting pan
Puncture the brisket all over with a marinating tool or large knife, The holes allow the seasoning to seep into the meat.
Generously spread the entire brisket with yellow mustard. You may need more than a cup of mustard, depending on how big the brisket is.
Sprinkle the rub all over the brisket and massage it in. The mustard acts as a binder for the rub. You can use any brand of BBQ rub that you like. Depending on the size of the container, you may not use all of the rub that is in the container. Use enough that satisfies you and that covers the brisket.
In a separate bowl, combine the ketchup, cider vinegar,brown sugar horseradish and pepper. Stir until well combined.
Pour the mixture sauce over the top of the brisket.
Top with onions.
Cover the brisket with aluminum foil.
Cook to the desired temperature. The length of time it takes to slow roast the brisket depends on its weight. A quick Google search will tell you exactly how long the brisket must bake according to how much it weighs. If you have a meat thermometer, you can check the inner temperature which should register 195 degrees.
When the brisket is done, allow it to rest 15-20 minutes before slicing it. The meat should be fall-apart tender when you cut it.