In a small jar, mix together the rub ingredients.
In a small bowl, stir together rub, brown sugar, paprika, salt, and cumin.
Trim fat from brisket, cut into 2-inch chunks, and rub pieces with spice mixture.
Lightly grease the slow cooker. Arrange the onion slices on the bottom of the cooker, then place the meat and garlic on top of the onions. Sprinkle with the cilantro.
Cover the pot and cook on low for 8 to 9 hours or until the brisket shreds easily with a fork. Serve on rolls. Use cooking liquid for sauce.