Mix 4 cups water with the seasonings;bring to a boil. Allow to cool.
Place brisket in a large crock or pan. Pour mixture over brisket. Arrange so that the brisket remains completely covered with brine. Cover with tin foil and refrigerate for 7 days.
Drain and rinse the brisket. Place brisket in a large pot or Dutch oven.
Add the onion and celery. Pour in 4 cups water. Bring to a boil. Reduce heat to a simmer. Simmer for 3 hours. Cool for 30 minutes in liquid.
Remove to a pastry sheet. Brush with mustard. Sprinkle with brown sugar.
Bake in a 350 oven for 30 minutes. Slice and serve.