Prepare the lamb
In a small bowl, mix 4 tsp. salt with the sugar and spices. Sprinkle all over the lamb, transfer to a large non-reactive dish, and cover; refrigerate for at least 4 hours and up to 2 days.
Make the sauce
Heat the oil in a medium (10-inch) sauté pan over medium heat until it’s shimmering. Add the onion, jalapeños, and 1 tsp. salt, and cook, stirring occasionally, until the onion softens and becomes translucent, about 8 minutes. Add the garlic, coriander, cumin, and red pepper flakes, and cook, stirring, until the garlic becomes fragrant, about 1 minute. Add the tomato paste, and cook, stirring, until the paste starts to darken, about 1 minute. Add 3/4 cup water and the vinegar; bring to a boil, then simmer until the onion is completely tender, about 10 minutes. Transfer to a food processor and purée. Let cool to room temperature, season to taste with more salt, pepper, and vinegar if needed. Set aside; or transfer to an airtight container and refrigerate for up to three days.
Grill the lamb
Tip:There are plenty of indicators used to check for doneness, but the best way to ensure you get your meat just to your liking is by taking its temperature with an instant-read thermometer. Just insert the thermometer in the thickest part of the meat, away from the bone.
For a gas grill, light the front burner to medium-high and the back burner(s) to medium-low. For a charcoal grill, light a medium-hot fire (400°F) with two-thirds of the coals banked to one side. Clean and oil the grill grates. Grill the lamb over the hotter part of the fire without moving it until it’s nicely browned, 6 to 8 minutes. Turn, move to the cooler part of the fire, and cook until an instant-read thermometer inserted into a thicker part of the lamb registers 135°F for medium-rare, 8 to 10 more minutes.
Transfer the lamb to a cutting board, tent with foil, and let rest for 10 minutes. Thinly slice across the grain, transfer to a large platter, and sprinkle with the cilantro. Serve with the sauce on the side.
Photo: Maren Caruso