Sprinkle the chicken breasts with salt and pepper and place in the bottom of the slow cooker. .
Place the tomatoes, onion, peppers, tomato paste and cumin into a blender or food processor and puree until smooth. Pour the pepper mixture over the chicken. Cover and cook 3-4 hours on Hugh or 6-8 hours on Low. The chicken is done when it reaches an internal temperature of 165°F on a meat thermometer.
Remove the chicken from the slow cooker. Cut into bite size chunks.
Top each tortilla with chicken garnish with salsa, tomatoes, lettuce and cheese if desired.
Reserve cooking liquid and serve as a sauce for the chicken.