Preheat the oven to 400 degrees F. Peel and core the tomatillos. Place the tomatillos and serrano pepper on a greased cookie sheet, and cook for 10 minutes.

Meanwhile, add the onion and garlic cloves to a medium saucepan and cover with water. Bring to a boil and cook for about 6 to 8 minutes, or until the onion is tender.

Strain the contents of the saucepan and place in a blender. Add the tomatillos and serrano, green chilies, oregano, cumin, salt and chicken broth. Blend until smooth. Serve with fajitas or your favorite Mexican dish.