Straight from Puebla, Mexico this is the way it's really made!
Place the tomatillos and jalapeno in a large saucepan. Add enough water so that the vegetables just start to float. Bring to a boil and cook until pale in color, about 3-5 minutes. Remove from heat.
Using a slotted spoon, transfer the tomatillos and ONE of the jalapeno quarters to a blender or food processor. Pulse until a chunky puree forms. Taste for heat. If you like your salsa hotter, add another piece of jalapeno. I added a whole pepper (seeds and all) and it's very spicy, so if you're sensitive to heat, don't add as much (or remove the seeds before boiling).
Add the onion, salt and cilantro and pulse several times to combine. Taste for seasoning adding more salt if desired.
Refrigerate salsa in a clean glass jar. Keeps for up to one week.