Combine half the dill with the seltzer, vinegar, 1 tsp coriander, and paprika, cumin, salt and pepper in a bowl. Add the lamb and onion wedges; cover and chill overnight.
Heat oil in a 4 qt saucepan over medium heat. Add the minced onion and garlic; cook for 6-8 minutes. Add the remaining coriander, plus tomato paste, chile flakes, prunes, salt, and pepper; cook until caramelized, 2-3 minutes. Add tomatoes, simmer. Cook until thick, 15 to 20 minutes. Stir in the remaining dill, plus cilantro, parsley, and juice.
Heat a charcoal grill or set a gas grill to high; bank the coals or turn off burner on one side. Thread the lamb and onions onto skewers. Grill over the coals, turning as needed, until cooked to desired doneness, 18-20 minutes for medium-rare. If outside burns before the lamb is cooked, move to the cooler side of the grill until done. Serve with the sauce.