In a 5 to 5-1/2-quart slow cooker, combine tomatoes, 3/4 cup of the salsa verde, cumin, garlic, salt and pepper. Add chicken, peppers and onion; stir to coat. Cover and cook on HIGH for 3-1/2 hours or LOW for 4-1/2 hours.
Remove chicken from slow cooker to a cutting board and let cool slightly.
Strain mixture over a large bowl or measuring cup; set liquid aside. Return vegetable mixture to slow cooker; stir in black beans and corn. Cover and cook on HIGH for 10 minutes.
Meanwhile, shred chicken and stir back into slow cooker with remaining 1/2 cup salsa verde and 1 cup of the reserved liquid (or more if desired).
Set aside half the mixture (3-1/2 cups). Heat shells according to package directions. Divide mixture evenly among shells; top each with 1 tablespoon cheese and chopped avocado slices. Freeze remaining half mixture for future use. Note: Mixture can be used in a quesadilla or wrapped in a flour tortilla.