For shish kebab, cut the meat into roughly 3-centimeter cubes of as even a size as possible; uniformity is the most important rule, because small pieces dry out while large ones remain raw. When threading the meat onto skewers, do not pack the pieces tightly; leave a few millimeters between them. Do not put meat and vegetables on the same skewer because their cooking times differ. These 3 rules determine the whole result.
The best cuts for shish kebab are lamb shoulder, leg of lamb, beef strip loin, and ribeye; they are tender enough and contain fat that gives the meat flavor. Tenderloin is too lean and can dry out easily on a skewer. Cut the meat across the grain into cubes about 3 centimeters in size; smaller pieces dry out in seconds over the coals, while larger pieces cannot cook through before the outside burns. Trim away membranes and sinew while cutting. The meat is easier to cut when it is refrigerator-cold, because cold meat holds together under the knife and makes neat cubes.
In traditional shish kebab, pieces of tail fat are threaded between the meat, for a clear reason: as the fat melts, it runs onto the meat below and slows it from drying out. A rough ratio is one piece of fat for every 3 to 4 pieces of meat. If you do not want fat, choose a fattier cut of meat. When using lean meat, marinating becomes important; olive oil, onion juice, and yogurt can partly serve this purpose and provide some protection for the meat’s surface.
The most common mistake when threading the skewers is packing the pieces tightly. On a tightly packed skewer, the surfaces facing each other never touch the coals; they do not brown, remain in their own steam, and turn gray. Leave a few millimeters between the pieces. Leave 3 to 4 centimeters of empty space at the two ends of the skewer so it can be turned easily. Pierce the pieces through their centers and across the grain; meat pierced near an edge can rotate as it cooks, slip off the skewer, and fall onto the coals.
Do not put meat and vegetables on the same skewer. Tomatoes and peppers cook much faster than meat and will have fallen apart by the time the meat is ready; in the reverse situation, the vegetables will come off at the right time while the meat is still raw. The right approach is to thread the vegetables onto separate skewers and turn the two skewers at different times on the grill. Onion is a partial exception because it cooks in about the same time as meat; even so, it works better cut into thick rings and threaded separately. Cook vegetable skewers over lower heat than the meat and near the edge of the coals so the inside softens before the outside burns.
The skewer itself also affects the result. Flat metal skewers hold the meat so the pieces do not rotate when turned; on a round skewer, the pieces spin around and the same side keeps facing the coals. If using wooden skewers, soak them in water for 30 minutes to keep them from burning. Cook the meat directly over the fire, turning it as each side browns. The target internal temperature is 63°C / 145°F for lamb and beef skewers and 74°C / 165°F for chicken skewers; insert the thermometer into the thickest cube. Taking the meat out of the refrigerator and letting it rest briefly before placing the skewers on the grill helps it reach the internal temperature more evenly.