Remove the corned beef brisket from the package.
To remove extra salt, soak the meat in cold water in the fridge, changing out the water every 30 minutes.
Three to four hours should be plenty of soak time to remove the extra salt.
Remove any excess fat on the non fat cap side of the brisket.
Coat the entire corned beef brisket with horseradish mustard then apply Jeff's original rub liberally to all sides.
Pat the rub down as it is applied rather than massaging it in.
Set up smoker for cooking at 225°F with hickory smoke.
Place the corned beef brisket on the smoker grate and smoke cook for 3 hours or until it reaches 140°F in the center of the thickest part.
Make a mustard sauce by mixing equal parts of Jeff's original barbecue sauce with horseradish mustard such as Dijon or Grey Poupon.
Create a pad of mustard in the bottom of a pan and lay the meat on the pad fat cap side up.
Brush more of the mustard sauce onto the fat cap and cover tightly with foil.
Place the meat back into the smoker at 225°F and continue cooking until it reaches 195°F in the thickest part.
Let the meat rest under foil for 30 minutes then slice pencil thick and serve.