Preheat the oven to 425 degrees.
Tear the paper husk off all of the tomatillos and discard the husk. Wash the tomatillos as they will be sticky. Take the white onion and cut it into 4 pieces. For the jalapeno, cut off the step and then quarter the jalapeno as shown in the photos.
On a cookie sheet, place the tomatillos, onion, and jalapeno pepper. Place in the oven at 425 degrees for 20 to 30 min. The vegetables should be roasted and soft. (See photo for more.)
Remove the roasted vegetables and cool, about 10 minutes. Transfer all of the vegetables to a blender. Add 1 chopped garlic clove, and the salt. Blend on high for 1 minute or until smooth. Add in the fresh chopped cilantro and blend well into the sauce. (This is optional for people who don't enjoy cilantro.)
Taste for seasoning and add additional salt if you like.
Store in Tupperware in the refrigerator until ready to use. The sauce will stay fresh for 5 days in the refrigerator and can also be frozen for 2 months. It's enough to make 1 large batch of our chicken or beef green enchiladas, which can be found on the blog.
