Place tomatillos in a nonreactive saucepan with enough water to cover. Bring to a boil. Simmer until tomatillos soften and begin to burst, about 10 minutes.
Drain tomatillos and place in a food processor or blender (you can also use a hand hold blender -- and do it right in the pot!) with onion, garlic, jalapeno peppers, cilantro, salt and pepper. Blend to desired consistency.