Place the tomatillos, garlic and 1/2 cup of the cooking liquid in the blender and puree. Add the chiles serranos, cilantro leaves, onion and salt, and puree again until smooth. Heat a tablespoon of oil in a pot over medium-high heat. Once it is hot, but not smoking, pour in the sauce and bring to a simmer. Let it simmer over medium heat for about 6 minutes, until it thickens and deepens in color. Taste for salt and add more if need be.