Preheat oven to 425 degrees F. Line a baking pan with foil paper, lay the Anaheim and poblano(if using) peppers on baking sheet, drizzle with a little oil. Roast in oven for 30 minutes, turning after 15 minutes. Remove from oven and cover with a clean kitchen towel, set aside to cool.
In a medium pot, combine the tomatillos, onions, garlic and serrano peppers, cover with water. If you prefer, you can roast the serranos as well. I enjoy the different flavors using different cooking methods. Cook on medium heat until the tomatillos go from bright green to an opaque olive color. Remove from heat and set aside.
Peel the blistered skins from the green chiles, remove the stems and seeds and chop roughly. Drain the liquid from the tomatillo mixture. Combine peppers, tomatillo mixture, cumin, 3 cups of chicken broth, 1/3 c masa harina and salt to taste. Blend on high until smooth. For smoother sauce, you can strain through a mesh strainer. If you have a power blender, no need to strain.
In a large sauce pan, heat oil to medium heat. When oil is hot, add the strained chile sauce and bring to a boil. Lower heat and taste salt. Continue cooking sauce for an additional 20 to 25 minutes, until sauce becomes thick.