For the sauce: Cook the tomatillos, garlic, and serrano pepper until the tomatillos change color.

Blend the tomatillos, garlic, serrano pepper, onion, and cilantro in a blender.

Pour the mixture into a skillet, add the sour cream, shredded cheese, salt, and pepper, and cook for 5 minutes.

To assemble the enchiladas, fill the tortillas with the chicken, roll them up, pour the sauce over them, and sprinkle with cheese.