Heat a medium saucepan on medium-high heat, add olive oil and chopped onions. Cook onions until they become slightly caramelized, about 3-4 minutes. Add the garlic and cook 2 minutes more.
Next add the chiles, tomatoes, broth, cumin, salt and pepper to the onion mixture. Bring to a boil, then reduce to a medium low and simmer uncovered for 15 minutes.
Gently whisk in cornstarch and cilantro and continue to simmer another 10 minutes until the sauce becomes thicker.
Transfer to a blender and puree sauce. Add salt and pepper to taste.
