FOR THE GUACAMOLE
5 to 6 medium (about 8 ounces) tomatillos, husked, rinsed and coarsely chopped
1 medium clove garlic
2 or 3 serrano chili peppers, stemmed but not seeded, then coarsely chopped
Leaves from 10 stems cilantro (about 1/2 cup packed)
1/2 cup water, plus more as needed
Flesh from 1 large ripe avocado
Kosher or sea salt
FOR THE TOSTADAS
About 1 pound fresh Mexican chorizo sausage, casings removed
About 2 dozen 6-inch corn tortillas, preferably somewhat dry and leathery
About 6 ounces queso fresco or other mild cheese, crumbled