1. Heat the broiler. Lay the whole tomatillos and serranos on a broiler pan or baking sheet (line it with heavy-duty foil for the greatest ease in cleaning and collecting the juices). Set the pan 4 inches below the broiler and let roast until the tomatillos are softened and splotchy black in places (the skins will split), about 5 minutes; your goal is to cook the tomatillos through while they roast, which means they'll have begun their transition from light bright green to olive color on the top side. With a pair of tongs, flip over the tomatillos and chiles and roast the other side for another 4 or 5 minutes or so. Set aside to cool.

2. Turn the oven down to 425 degrees. Separate the onion into rings and, on a similar pan or baking sheet, combine them with the garlic. Set in the oven. Stir carefully every couple of minutes, until the onions are beautifully roasted (they're going to look rather wilted and translucent, even have a touch of char on some of the edges); the garlic should feel soft and look browned in spots. Total roasting time will be about 15 minutes. (If you like smoky flavors in your salsa, try roasting the onion and garlic on a perforated grilling pan over a moderately low charcoal fire.) Cool to room temperature.

3. In a food processor, pulse the serranos (no need to peel or seed them) with the onion-garlic mixture until moderately finely chopped, scraping everything down with a spatula as needed to keep it all moving. Scoop into a big bowl, then, without washing the processor, coarsely puree the tomatillos and their juice. Stir them into the bowl. (If you're making the largest quantity, you'll have to do the tomatillos in two batches.) Stir in enough water to give the salsa an easily spoonable consistency-in Mexico they like this salsa spicy and pretty runny, the kind of salsa you dribble on rather than scoop up with chips. Stir in the cilantro.

4. Taste and season highly with salt. Taste again and, if you wish, add just enough sugar to take the edge off the tomatillos' very bright tanginess. If you're planning to use your salsa right away, simply pour it into a bowl and it's ready, or refrigerate it and use within 5 days.