Remove husks from tomatillos. Rinse.
Cut the onion into large pieces. Remove stems from chilies and tear into pieces.
Place tomatillos, onion, chilies and garlic in a large pot. Fill the pot with water until the water is ¼" below the top of the vegetables. (About 4 cups)
Bring to a boil, then reduce heat to a simmer. Simmer for 10 minutes.
Pour contents of the pot into a blender. Add cilantro. Blend until smooth.
Wipe out the inside of the pot. Add 2 tbsp oil and heat over medium heat. Pour in a small amount of the blended mixture (about ¼ cup) and cook for about 1 minute.
Pour remaining mixture into pot. Bring to a simmer and let cook for about 20-30 minutes or until desired consistency is reached.
Remove from heat and stir in salt to taste.