Remove the papery husks from the Tomatillos, rinse under cold water to remove any stickiness, and pat dry. Trim the stems from the Serrano Chiles (leave seeds for more heat or remove for milder salsa), peel and quarter the White Onion, and peel 2–4 cloves of Garlic.
Heat a dry cast-iron skillet or griddle over medium-high heat. Roast the prepared tomatillos, chiles, onion quarters, and garlic cloves, turning occasionally, until the skins are blistered and there's a little char all over (about 8–10 minutes); you can also broil them on a sheet pan if you prefer.
Let the roasted vegetables cool slightly, then transfer them to a blender or food processor. Add a generous handful of Cilantro, about 1 teaspoon of Salt, 1–2 tablespoons of Lime Juice (or to taste), and 2–4 tablespoons of Water to help the blender move.
Pulse or blend to your desired consistency—leave it a bit chunky for salsa or blend until smooth for a sauce. Taste and adjust seasoning, adding more salt or lime juice as needed.
If time allows, chill the salsa for 20–30 minutes to let the flavors meld; otherwise serve at room temperature. This salsa is fantastic with chips, tacos, grilled proteins, or stirred into soups and stews.
Store leftovers in an airtight container in the refrigerator for up to 5 days; flavors will develop further after sitting.
