Two versions I found.
VERSION 1
Ingredients
1 1/2 pounds tomatillos, husks removed, split in half (680g; about 10 medium)
1 medium white onion, peeled and split in half (about 6 ounces; 170g)
2 to 4 Serrano or jalapeño chilies (adjust according to spice tolerance, remove seeds and ribs for milder spice), split in half
10 to 15 sprigs cilantro, tough lower stems discarded
1 tablespoon vegetable oil
Kosher salt
Instructions
1. Adjust oven rack to 4 inches below broiler and preheat broiler to high. Place tomatillos, onion, and chilies on a foil-lined rimmed baking sheet. Broil until darkly charred and blackened on top and tomatillos are completely tender, 6 to 12 minutes.
2. Transfer vegetables and their juice to a blender, food processor, or the cup of an immersion blender. Add half of cilantro. Blend in pulses until a rough puree is formed.
3. Heat oil in a medium saucepan over high heat until shimmering. Pour salsa into the hot oil all at once (it will steam and sputter). Immediately start stirring and continue to cook, stirring, until salsa is darkened and thick enough to coat the back of the spoon, about 3 minutes. Remove from heat.
4. Finely chop remaining cilantro and stir into salsa. Season to taste with salt. Let cool, then serve. Salsa can be stored in the refrigerator for up to 5 days.
VERSION 2
Ingredients
8 ounces (3 to 4 medium)tomatillos, husked and rinsed
Fresh hot green chiles to taste (1 or 2 serranos or 1 jalapeno), stemmed
2 large garlic cloves, peeled
6 sprigs of fresh cilantro (thick bottom stems cut off), roughly chopped
1 small white onion, finely chopped
Salt
Instructions
Roast the tomatillos, chile(s) and garlic on a rimmed baking sheet 4 inches below a very hot broiler, until blotchy black and softening (they’ll be turning from lime green to olive), about 5 minutes. Flip them over and roast the other side. Cool, then transfer everything to a blender, including all the delicious juice the tomatillos have exuded during roasting. Add the cilantro and 1/4 cup water, then blend to a coarse puree. Scoop into a serving dish. Rinse the onion under cold water, then shake to remove excess moisture. Stir into the salsa and season with salt, usually 1/2 teaspoon.