Prepare the tomatillos: Remove papery husks from tomatillos and place them on a baking sheet. Position oven rack 4 inches below broiler and preheat broiler.

Add garlic and pepper: Place unpeeled garlic cloves on the baking sheet. Remove stem from jalapeño and place cut-side down alongside tomatillos.

Roast the vegetables: Broil for 6 minutes, then flip everything and broil for another 6 minutes until tomatillos are darkly roasted and softened.

Cool and prepare: Let roasted vegetables cool completely. Peel garlic cloves by gently pressing – skins should slip off easily.

Blend the base: Add roasted tomatillos with all their juices, peeled garlic, and roasted jalapeño to blender. Pulse until you achieve a slightly chunky consistency – not completely smooth.

Season and adjust: Pour salsa into serving bowl. Add salt to taste and thin with water if desired for a more traditional Mexican consistency.

Prepare onion: Dice white onion into small pieces (less than 1/4 inch). Rinse under cold water and drain well to remove harsh bite.

Finish with aromatics: Stir in rinsed onion. Chop cilantro leaves and tender stems together, then fold into salsa.

Final seasoning: Taste and adjust salt as needed. Let flavors meld for 10 minutes before serving.