The key to juicy Shashlyk is using lamb with a bit of fat (or alternating with pieces of fat) and a long, thorough marinade. The vinegar helps tenderize the meat fibers, and the onion imparts its aroma.
When grilling, the meat is cooked on flat, wide skewers (shampurov), not round ones, which ensures even cooking.
Cut the meat into uniform cubes (about 4x4 cm).
Grate or blend the onion into a paste and place it in cheesecloth or a sieve. Squeeze out the maximum juice into a bowl. Discard the onion pulp or use it in another dish, but not on the shashlyk.
Add vinegar, water, salt, pepper, and cumin to the onion juice. Mix well.
Place the meat into the marinade. Mix thoroughly by hand to coat every piece.
Cover with plastic wrap and refrigerate for a minimum of 12 hours, ideally 16-24 hours.
Thread the meat onto flat skewers (shampurov). The pieces should be close together. If using fat, alternate a piece of meat and a piece of fat.
Heat the grill to medium-hot coals (no flames).
Place the skewers on the grill. Grill for 20-25 minutes, turning regularly to cook the meat evenly on all sides. The meat is done when it is browned on the outside and no longer raw in the center (internal temperature 65 °C / 150 °F).
Slide the meat off the skewers or serve directly on them.
Shashlyk is traditionally served with fresh onion sliced into rings, sprinkled with dill and coriander, and with Lepyoška (traditional Central Asian bread).