Brown chicken breasts in a large skillet over medium-high heat for about 2 minutes per side. Transfer chicken breasts to the slow cooker.
Stir all other ingredients together except cilantro and lime slices, mixing until paste is completely dissolved. Pour into crockpot over chicken. Fold in onion, mushrooms and pepper and stir well.
Cook on low for 6 hours.
Shred chicken into chunky pieces and stir back into the soup before serving.