You will need a large skillet or heavy saucepan and either a blender or food processor.
Heat the oil in a large heavy skillet or saucepan over medium high heat. Add garlic and onion and cook, stirring occasionally, until onion is transparent. Add chiles and cook for one minute more. Add tomatillos, then pour in broth and add sugar and salt. Bring to a boil, lower heat and gently simmer uncovered for thirty minutes. Remove from heat and let cool. Taste for salt and adjust if necessary. You can either mash the tomatillos with a fork to produce a coarse-textured salsa, or transfer the sauce to a food processor or blender to be briefly processed to a smooth texture. Transfer sauce to a serving dish and just before serving, stir in coriander leaf.