In a small bowl, combine the 1/2 cup sugar 1/4 cup kosher salt, 4-6 garlic cloves, 2 tbsp. freshly ground black pepper, and 1 tsp. ground cumin for the dry rub and mix well. Rub the seasoning over the 4 - 5 lb. beef brisket. Wrap the brisket in plastic wrap and place it on a large sheet pan. Refrigerate for at least 24 and up to 48 hours.
Heat the 3 tablespoons vegetable oil in a large heavy skillet over medium-high heat. Wipe off the excess brine from the meat and add it to the skillet. Sear for 5-7 minutes on each side, until the brisket is dark golden brown.
Transfer the meat to a lightly greased 6 quart slow cooker. Add the 1 medium onion, 1 garlic clove, 2 teaspoon chili powder, 1 teaspoon ground coriander, 2 teaspoons ground cumin, 1/4 cup apple cider vinegar, 1 14 oz. can diced tomatoes, undrained, 2 chipotles in adobo, 2 dried bay leaves and 1/4 cup molasses to the slow cooker and stir to combine.
Cover and cook on low for 8-10 hours, or until the brisket is tender. Remove to a baking pan and cover to keep warm.
Transfer about 2 cups of the cooking liquid to a medium saucepan. Bring to a boil over medium-high heat, then simmer, uncovered, until liquid is reduced by half.
Shred the beef with two forks and pour in the reduced sauce. Toss to combine and serve immediately.