The salsa can be refrigerated for up to 5 days.
1½  pounds tomatillos, husked and rinsed
2   cups water
½   medium white onion, chopped
3   large garlic cloves, minced
2   large scallions, chopped
½   cup chopped cilantro
¼   cup chopped epazote or 1 tablespoon chopped oregano
1   teaspoon cumin seeds
fine sea salt
Combine all the ingredients except the salt in a large saucepan and bring to a boil. Add a generous pinch of salt and simmer over moderate heat until the onion is tender, about 20 minutes. Remove from the heat and coarsely mash the salsa with a potato masher. Let cool completely, season with salt and serve.
Enrique Olvera, Pujol, Mexico City