Remove the papery husks from the tomatillos. Rinse to take away the sticky residue—you will know what I mean when you feel it. Place the tomatillos in a saucepan and cover with water. Cook until the tomatillos are just soft, about 5 minutes.

Alternatively, and more traditionally, char the tomatillos in a hot cast-iron skillet until they are soft. (Mexican cooks often roast the onion and garlic at the same time. The char brings smoky depth, but if you want to keep it simple, skip this additional step—your salsa will still be terrific.)

Place the softened tomatillos and about ½ cup water in a blender jar.

Cut the top off the serrano chiles and cut them in half lengthwise. Scrape the seeds away unless you like maximum picante heat. (Grapefruit spoons are the perfect utensils for holding chiles while you scrape.) Chop coarsely and add to blender. Add the onion, garlic, cilantro, and salt.

Puree until the sauce is smooth. Taste and add more chile and/or salt if needed.