Preheat oven to 400 degrees. Line a baking sheet with parchment. Spray a second baking sheet with a little oil.
Add tomatillos, garlic and jalapeno to the baking sheet and roast for 25 minutes until starting to char and brown. Remove skin from garlic and then blend all roasted ingredients with remaining sauce ingredients (through avocado) until smooth. Taste and adjust seasoning. Chill.
Season chicken breasts with salt and pepper. Heat a skillet with a little oil over medium heat. Cook chicken for 6-8 minutes until it reaches 165 degrees. Allow chicken to rest for 5 minutes and then shred with 2 forks. Run your knife through the chicken until small, like browned ground beef.
In a bowl, combine chicken with salsa and cheese. Adjust the amount to your personal preference.
Warm corn tortillas in a tortilla warmer or over a burner. Working quickly, while the tortillas are warm, add a couple tablespoons of chicken mixture to the center of the tortillas in a line. Roll up and place on the greased baking sheet seam side down. Spray tops lightly with oil.
Bake at 400 degrees for 15 minutes until golden.
Serve with tomatillo avocado sauce for dipping.