To make rub, in a small food processor or bullet blender, finely chop garlic, olive oil, and worcestershire sauce. Add salt, brown sugar, dry mustard, black pepper, cumin, paprika, and onion powder and blend until smooth.
Rub the four brisket pieces with the rub. Cover or wrap with plastic and let meat rest about 1 hour, until it is room temperature.
Add onions to the bottom of slow cooker. Add beer and water. Place brisket pieces, fat side up, on top. Cover and cook 8-9 hours on low or 5-6 hours on high, until beef is fork tender.
Preheat oven on broil.
Move onions and braising liquid to a roasting pan and top with beef brisket. Broil 5-10 minutes, until browned.
Slice or shred brisket and add to a serving dish. Using tongs or a slotted spoon, add onions from roasting pan into brisket and combine. Add braising liquid as needed. Add salt to taste (we find that additional salt is not needed).
For gravy, add about 2-3 cups remaining braising liquid to a medium saucepan and bring to a boil on medium high heat.
Mix together equal parts cornstarch and water to form a slurry. while whisking the braising liquid, add the slurry slowly, until the gravy reaches the desired consistency. (You may not need all of the slurry.)
*For smaller pieces of shredded beef, you may want to cut the beef into smaller pieces, about 4 inches. Cook on high for 7-8 hours and low for 4-5 hours.This brisket is perfect to cook ahead. Braise a day ahead. After moving to the roasting pan, cool, cover and refrigerate. To reheat, bake at 325F for 90 minutes. Broil as directed. We’ve done this many times and it is perfect every time.If doubling the meat, do not double salt in the rub.Nutrition information does not include optional gravy.