1Combine half the dill with the club soda, vinegar, 1 tsp. coriander, and paprika, cumin, salt, and pepper in a bowl. Add lamb and onion wedges; cover and refrigerate overnight.
2Heat oil in a pan over medium heat. Add minced onion and garlic; cook 6-8 minutes. Add remaining coriander, plus tomato paste, red pepper flakes, prunes, and salt, and pepper to taste; cook until caramelized, 2-3 minutes. Add tomatoes; simmer until thick, 15-20 minutes. Stir in remaining dill, plus cilantro, parsley, and lemon juice.
3Heat grill on high. Bank coals or turn off one burner. Alternate lamb and onions on skewer. Cook over heat 18-20 minutes for rare. If outside is cooking too fast, move to a cooler area.
4Remove from skewers and serve with tomato prune sauce.