Step 1: If using a charcoal grill:
Light a chimney of coals and rake them out in a singer layer. Lay the onion on the embers and roast until charred on all sides and easy to pierce with a skewer, 8 to 12 minutes in all, turning with tongs. Transfer to a foil pan. Lay the tomatoes on the embers and roast the same way, 6 to 8 minutes in all. Transfer to the pan. Lay the habaneros on the embers and roast the same way, 2 to 4 minutes in all. Transfer to the pan.

If using a gas grill:
Preheat it to screaming hot. Grill the vegetables to char the skins.

Step 2: Scrape off the burnt skins with a paring knife. You don’t need to get off every last bita few specs of black will add flavor. For a milder (but still ferocious salsa), seed the chiles. Cut the vegetables into 1-inch pieces. Finely chop or coarsely puree in a food processor. Work in the cilantro, sour orange juice, and salt, running the machine in short bursts. Correct the seasoning, adding sour orange juice or salt to taste: the salsa should scream with flavor.