Husk and wash the tomatillos, wash the jalapeños, and peel the garlic.
Roast over a medium flame on a gas grill or under the broiler, turning the veggies until they are lightly charred all over.
Remove to a bowl and allow to cool.
Seed the serrano pepper, halve it lengthwise, then mince it. Dice the onion to a ¼” dice. Wash the cilantro, then shake the water out of it and dry it with a clean towel. Place on a cutting board and chop the leaves and small stems down to where there are few to no leaves.
Place the cooled veggies in a blender, removing the stems from the jalapeños. Add the onions, then blend until it is chunky-smooth.
Pour into a bowl and stir in the salt and cilantro. Taste before adding the serrano as you may not want to use the whole thing.
Chill for an hour or so before serving or serve warm.