Place one sheet of foil on flat surface. Lay brisket across the foil. Take dry ingredient(mix together in bowl)and rub real good over the top of the brisket.
Place the other sheet of foil on a flat surface. Lay sliced onion across the foil in the shape of the meat.
Place the brisket top down across the onion. Rub remaining dry ingredients on fat side. Squirt mustard across the brisket. Rub evenly all over.
Place the other sheet of foil on top and crimp edge together. Place the whole thing on an oven roaster with a lid. Make sure not to turn the brisket over. You want the fat side up because the fat is going to cook away and all the seasoned juices will soak in the meat as it cooks.