In a pot large enough to hold the soup, cook the onion and garlic in the oil over medium heat, stirring regularly, for about 10 minutes, or until the onion softens and is shiny but does not brown.
Add the chiles to the onion. If using fresh tomatillos, cut them in half before you add them to the pot. If using canned tomatillos, add them whole. Add the broth, cover, and simmer for 10 min