Heat the oil in a medium saucepan over medium heat. Add the onions and cook until lightly golden brown. Add the garlic and cook for 30 seconds. Add the chicken broth, 3/4 cup water, the ketchup, vinegar, ginger, cayenne, dates, cloves, cinnamon stick and honey to taste and cook until the dates are very soft and the mixture has reduced slightly.
Transfer to a blender and blend until smooth. Add the chopped cilantro and salt and pepper to taste and pulse to incorporate. Transfer to a bowl and let cool. Remove 1/4 cup for garnish.
Heat a charcoal or gas grill to medium-high for direct grilling.
Brush the lamb with oil on both sides and sprinkle with salt and pepper. Grill, brushing with the glaze and turning once or twice, until cooked to medium-rare, about 10 minutes total. Remove and let rest before slicing.
Place the lamb slices on a platter, drizzle with the reserved glaze and garnish with cilantro leaves.
When blending hot liquid, first let it cool for five minutes or so, then transfer it to a blender, filling only halfway. Put the lid on, leaving one corner open. Cover the lid with a kitchen towel to catch splatters, and pulse until smooth.