Put onion, garlic, spices, herbs, vinegar, and salt in a food processor and process into a smooth, thickish marinade.
In a bowl, combine lamb and marinade; turn to coat thoroughly. Cover and chill overnight.
Preheat a grill until very hot (550° to 600°). Slide about 4 lamb cubes onto each skewer, shaking off excess marinade. Oil grill well with a wad of oiled paper towels, then grill skewers (covered if using gas), turning once halfway through, until medium rare (cut to test), 6 to 10 minutes; they will continue to cook to medium off the heat. Season with salt.
Lay skewers on a platter, top with cilantro sprigs, and serve with plum sauce.
*Find fenugreek seeds in the spice aisle; grind in a spice grinder or clean coffee grinder.