1. In a bowl, combine onion powder, garlic powder, paprika, salt, pepper, and celery salt; mix thoroughly. Dredge the brisket in this spice mixture, making sure every part of it is well covered. Place in a deep dish covered with aluminum foil, and refrigerate for 1 or 2 days.
2. Heat 2 tablespoons of the corn oil in a large skillet, and brown the meat on both sides.
3. Transfer brisket to a Dutch oven, add ½ cup water, cover, and simmer for 1 hour.
4. While meat is simmering, heat remaining 2 tablespoons corn oil in a large skillet, and sauté onions, stirring occasionally. When the onions are nicely browned, add garlic, which browns quickly.
5. Add onions and garlic to brisket pot. Cover, and continue simmering for 2 ½ hours, or until meat is fully cooked. To test for doneness, stick a fork in the leaner end of the brisket; when there is a slight pull on the fork as it is removed from the meat, it is done. Cook longer if necessary.
6. Remove brisket to a plate, and trim all visible fat. Then place the brisket (with what was the fat side down) on a cutting board, and carve thin slices across the grain (the muscle lines of the brisket) with a sharp, thin-bladed knife. Serve hot with gravy from the pot or cold in sandwiches.