Stir together spice rub ingredients in a small bowl.
Rub spice mixture onto the surface of the roast.
Heat the olive oil in a large skillet over medium-high heat.
Add the beef, and cook until browned on all sides.
Place beef in slow cooker insert.
Reduce heat to medium.
Add garlic and onion to now-empty skillet, and cook 3-4 minutes, stirring regularly, until onions have just softened.
Pour onions and garlic around roast.
Add remaining ingredients to slow cooker.
Pour in enough water so that the beef is about 2/3 covered with liquid.
Cover, and cook on LOW for 8-10 hours. Beef will be done cooking when it is fork tender.
Remove the beef, and shred with 2 forks.
Turn slow cooker to HIGH and let the sauce cook with the lid OFF for about 20 minutes to thicken the sauce.
Serve beef topped with sauce, or return beef to slow cooker and toss beef in sauce.
Notes
*I prefer ancho chile powder because of it's milder, smokier flavor.**Chuck roast or a similar cut of meat will also work in this recipe.***Nutrition values are estimates.