Place the tomatillos along with the garlic cloves and the chiles serrranos in a pot and cover with water. Place over high heat until it comes to a boil. Simmer at medium for about 10 minutes, or until tomatillos change their color from a bright to a pale green, are cooked through and are soft but are not coming apart.
Place tomatillos, garlic and chile (you may add one chile first) and 1/2 cup of the cooking liquid in the blender and puree until smooth. Incorporate the cilantro leaves, onion and salt and process again. Taste for salt and add more if need be. Also taste for heat, you may add the other chile in pieces until you reach your desired heat level.
Heat the oil in a pot over medium heat. Once it is hot but not smoking, pour in the sauce and bring it to a boil. Simmer over medium heat for 6 to 7 minutes, until it thickens a bit and deepens its flavor and color. Turn off the heat.
Once it cools down, you may store it in a closed container in the refrigerator for weeks. However, the heat level of the sauce will diminish as the days go by.