I like to make this salsa with raw tomatillos. If you prefer, you could cook them in boiling salted water for about 10 minutes, drain and then proceed.
1 pound tomatillos, paper skin removed and washed, quartered
2 jalapenos, seeded and ribs removed if you want to tame the heat
1 cup cilantro
1 garlic clove
1 small onion, coarsely cut
salt to taste
Place all in a food processor and pulse. The mixture should have some texture.