Trim the meat of all fat, cut into 2 1/2 cm (1 in) cubes. Add liquid saffron, grated onion, salt, pepper, lemon juice to the yogurt. Marinade the lamb in the mixture for at least 3 hours, preferably overnight. Cut up the green peppers in 2 1/2 cm (1 in) squares. Cut the onions into quarters. Thread the meat, pepper, onions and tomatoes alternately on to a fine skewer and grill over glowing coals, turning frequently.
When cooked, push the kebabs off the skewers on to a warmed dish and serve immediately.