Cut the lamb and beef into 25 mm (1 in) cubes. Cut the kidneys in half, skin them and remove the cores; cut each half into 4 pieces. Place the pieces of meat in a large shallow dish. Mix together the olive oil, lemon juice, finely chopped garlic and herbs and pour over the meat. Leave in a cool place for 1 hour, turning occasionally.

Heat the grill to medium-high. Meanwhile, remove the grid from the grill pan and line the pan with aluminium foil. Thread the pieces of meat onto 4 greased metal skewers, alternating with the mushroom caps and parboiled onions.

Lay the skewers across the grill pan. Grill about 7.5 cm (3 in) from the heat, basting with the marinade and turning from time to time for about 20 minutes, until the meat is cooked. Garnish with rosemary and lemon and serve.