This is basically a Mexican salsa verde, only with a heavier hit of epazote. It's loosely based on a similar salsa by the great Mexican chef Enrique Olvera; I roast my vegetables, where Olvera simmers his. As you see in the picture, this is rather thick, but feel free to thin it with water if you'd like.
Turn your broiler on high. Arrange the tomatillos, onion and garlic on a baking sheet and set under the broiler until about halfway charred. Keep an eye on it, as this can take anywhere from 5 minutes for dryish tomatillos to 15 for wet ones.
Meanwhile, char the serranos over a gas burner, holding them with tongs. This is a better method for charring your serranos because they don't cook fully this way, but if this bothers you, put the serranos under the broiler with everything else.
Remove the garlic and peel it when it's cool enough to touch. Remove the skins and seeds from the serranos. Move everything into a blender, along with all the other ingredients. Puree. Add salt to taste.