Preheat oven to 375°F. Spray/grease a 9×13 baking dish, set aside.
In a medium sauce pan, melt butter over medium-high heat. Sauté serrano pepper and garlic, for 1 minute. Stir in flour, constantly stirring, and cook for 2 minutes.
Whisk in the chicken broth, cumin, salt and pepper and bring to a low boil. Once boiling, whisk in the sour cream, salsa verde and cilantro. Remove from heat. Spread ¾ cup sauce evenly on the bottom of the prepared baking dish.
Add chicken, Monterey jack cheese, onion and avocado to the center of each tortilla and roll, placing seam-side down in the baking dish.
Pour the remaining sauce over the enchiladas. Top with leftover cheese and bake for 15 to 20 minutes, until bubbling. Let rest for 5 minutes, then serve!
