Put the meat cubes, and lemon and onion slices in a large bowl. Remove the leaves from the dill and parsley stems, chop up the hard stems (the leaves will be used later on), and toss them into the bowl. Add the salt, black pepper, red pepper flakes and 5 tablespoons of vinegar. Mix thoroughly with your hands. Cover the bowl, and allow to marinate overnight in a cool place.
Build a hot charcoal fire in a grill. Thread the meat cubes tightly onto the skewers, and grill, turning frequently, until the meat is cooked throughout. If your fire gets too strong, drizzle some vinegar-added water over the skewers from time to time.
Chop up the dill and parsley leaves, and sprinkle them over the kebabs along with the chopped spring onion. Drizzle with 1 tablespoon of vinegar. Serve with flatbread or rice pilaf.