Preheat oven to 350 degrees F. Lay brisket with the fat pad on the top in the dish. Wash potatoes and quarter them, place around meat.
Sprinkle seasoning packet over top of meat. Pour cup of beer or liquid over the top of everything. Cover your pan with foil tightly.
Bake for 3 hours for a 2.5-3 lbs size or about 1 hour 10 minutes per pound.
Then remove foil, whisk up mustard sauce and pour over the top of the meat, make sure is covers the whole top and is dripping on to the sides. Put back into oven uncovered for 15 minutes.
Remove, put foil on top to keep warm and allow to rest for 10 minutes before slicing.
Serve with tender potatoes seasoned with salt and pepper and enjoy.