Arrange an oven rack to the top position and heat the broiler. Meanwhile, line a rimmed baking sheet with aluminum foil and prepare the vegetables.
Peel off any papery husks from 1 pound tomatillos. Rinse off the sticky coating and any dirt under cold water, then dry with paper towels or a clean kitchen towel. Halve the tomatillos and place cut-side down on the baking sheet.
Place 3 peeled and smashed garlic cloves on the baking sheet. Place the halved serrano or jalapeño pepper cut-side down on the baking sheet.
Broil until the tomatillos and pepper skins start to blacken and blister, checking every few minutes and rotating the baking sheet as needed, 5 to 7 minutes total. If the garlic is charred before the tomatillos are ready, remove it.
Set the baking sheet aside until cooled to room temperature, about 15 minutes. Place ½ cup diced white onion in a fine-mesh strainer and rinse under cold water. Drain well.
Transfer the tomatillos, garlic, and pepper to a blender. Add the onion, ½ cup coarsely chopped fresh cilantro, 1 teaspoon kosher salt, and 2 tablespoons water. Blend into your desired consistency.
