Mix the dry rub ingredients together in a small bowl. Divide in half and set aside.
Using 1/2 of the dry rub, rub both sides of the brisket with it. Save the other half of the rub for adding to the brisket at the end of the cooking time.
Cut brisket in half crosswise so that it will fit in the crockpot.
Place some of the onions and garlic on the bottom of the crock-pot. Lay the brisket on top of the onions and garlic. Scatter the remaining onions and garlic over the brisket. I separated some of the onion layers and broke off pieces of the garlic so that there would be more pieces for flavoring the meat. I also put some of the onion and garlic pieces between the overlapping pieces of brisket.
Pour 1/2 cup water around edges. Put lid on crock-pot and cook brisket on low for 8-10 hours.
Remove brisket, allow to cool for a few minutes, then shred with two forks. The brisket literally falls apart at this point.
If desired, do a quick rough chop of the meat to cut it into smaller bite-sized pieces. Remove the onions and garlic from crock-pot. Leave about 2 cups of the juices in the bottom ( it doesn’t have to be exact, but don’t leave more than 2 cups of juices behind ).
Return meat to crock-pot and sprinkle with remaining rub. Toss to coat well. Turn crock-pot to high and allow the meat to get hot again. In the meantime prepare the accompaniments for the tacos. Taste and adjust seasonings prior to serving. If you prefer more heat, add more chipotle chile powder. You may also prefer a saltier flavor, so add a little more salt.
May be served directly from crock-pot or removed, drained, to a separate serving dish. Please note: the cooking liquid helps to keep the meat from drying out. Serve with flour tortillas and taco toppings such as diced tomatoes, shredded lettuce or cabbage, black olives, salsa, grated cheese, sour cream, or guacamole.
This makes a lot of meat for tacos, enchiladas, taco salads, etc. Left-over brisket can be portioned out and easily frozen in air-tight containers for later use.