Place the tomatillos,garlic cloves and the serannos into your dutch oven, cover with water about 1 inch above the vegetables.
Place over high heat until it comes to a boil. Turn heat to medium and simmer 10-12 minutes until the tomatillos change their color from bright green to a pale green and are soft.
Once soft and the color has changed, put the tomatillos, garlic and serranos and ½ cup of the cooking liquid into a blender. ( you may start with 1 serrano and add 2nd if you prefer it spicier) Puree until smooth. Once smooth, add in the cilatro onion and salt and blend again. Taste for salt and add more if needed.
Pour out the cooking liquid from your dutch oven and heat on stove over medium heat adding the oil, once hot but not smoking pour in the salsa from the blender and bring to a boil. Simmer over medium heat for 8-10 minutes until it thickens slightly and turns darker in color.
Turn off heat and allow to cool. You can store in air tight container in the refrigerator for about 1-2 weeks. I put it into a squeeze bottle.