Whisk together the chili powder, salt, pepper, brown sugar, cumin, oregano, cinnamon and cloves in a small bowl.
Pat the roast dry with paper towels and rub the spice mix into the roast on all sides.
Heat the vegetable oil in a large frying pan on medium high heat. Place the roast in the pan and brown on all sides.
While browning the roast, line the slow cooker with the onions and garlic.
Remove the roast from the pan and place in the slow cooker on top of the onions and garlic.
Pour the chicken broth over the roast.
Cover and cook on low for 8 hours.
Remove the roast from the slow cooker and place on a cutting board. Use two forks to pull the meat apart and into bite sized shreds. Return the meat to the slow cooker and stir to mix with the gravy.
Serve with tortillas, lettuce, cilantro and cheese.