Combine the first four ingredients in a small bowl. Rub into the lamb, coating all sides evenly. Cover and let rest 30 minutes.
While the lamb rest, prepare the sauce. Heat the oil in a medium saucepan until it shimmers. Add the onion and jalapeño. Cook until translucent, about 5 minutes. Add the spices and cook for 1 minute. Add the tomato paste and cook for one more minute. Stir in 3/4 cup water and Vinegar. Bring to a boil and cook for 15 minutes. Transfer to a food processor or blender and purée. Let cool to room temperature.
Heat a grill to medium-high. Grill the lamb for 6-8 minutes on one side, or until it is deep brown. Flip the lamb over and turn off the burner directly underneath. Grill 8-10 more minutes. Remove lamb from heat and let rest for 10 minutes. Sprinkle with cilantro. Slice across the grain and serve with sauce.