1. Combine spice mixture. Wipe brisket with a damp paper towel and seasoned brisket with spice mixture and marinate overnight, when ready to cook, dredge the meat in the flour.
2. Heat the oil in a large roasting pan with a cover.
3. Brown the meat, starting fat side down, on both sides to seal in the juices.
4. Pour the vinegar over the meat, letting it drip onto the pan, add all the remaining ingredients except the onions.
5. Bring to a slow boil, basting the meat until the ingredients are blended.
6. Pre-heat the oven to 350F. Cover the pan and place in the oven to braise for 1 hour.
7. Meanwhile, wash the carrots and the potatoes. Cut the carrots into 1-inch pieces. Leave the smaller potatoes whole and cut the larger potatoes in half or in thirds. Place the potatoes in a bowl and cover with cold water until ready to add to the brisket.
8. After 1-hour in the oven, remove the brisket and add the carrots and potatoes. Cover and return to the oven for another hour. Continue to baste occasionally, adding a small amount of boiling water as needed.
9. Peel and slice the onions into 1/2-inch slices. Set aside.
10. After the second hour in the oven, remove the brisket and lay the onions in a single layer over the top of the meat. Extra onions may be placed around the meat. Baste with the gravy.
11. Cover the pan and return to the oven for about 30-45 minutes or until the onions are tender. Taste for seasoning and add salt and pepper as needed.
12. Let meat stand 1/2-hour, then slice across the grain.