Directions for Salsa Verde:
Bring 6 cups water to a boil in a large saucepan over medium heat. Add serrano chiles and tomatillos. Boil for 8 minutes. Turn heat off. Remove tomatillos but leave serrano chiles for 5 minutes more. Drain and slice off serrano chile stems.
Roast poblano peppers over an open flame until charred on all sides. Transfer to a plastic bag and seal. Steam for 10 minutes. Carefully open bag and peel charred skin by scraping with a knife. Slice open and remove seeds.
Heat vegetable oil in a large saucepan over medium heat. Add garlic cloves and onion and stir until onion softens; about 4 minutes.
Add serranos, tomatillos, poblanos, cilantro, sautéed garlic and onion and chicken broth to blender. Blend until smooth.
Heat the same saucepan where garlic cloves and onions were fried over medium-low heat. Add green sauce and season with salt. Cook for 15 minutes, stirring frequently. Remove from heat, cover and keep warm.