Place all the of spices (cumin, garlic powder, smoked paprika, oregano, salt, pepper/cayenne) in the bottom of a slow cooker insert and mix well. Add chicken breasts and evenly coat with spice mixture.
Add salsa, beer and sliced onions to the slow cooker then heat on Low for 6 hours or High for 3 hours. Shred chicken with two forks to serve. Ideal in tacos, salads, burrito bowls and more.