1) Cut the peppers into 1 inch pieces. Then peel and cut the onion and into wedges with the root end intact.
2) Thread the onion wedges, tomatoes mushrooms and peppers with alternate colours on to the skewers.
3) In a small bowl, combine the oil, garlic and herbs. Then baste the kebabs with the mixture.
4) Grill the kebabs over medium hot coals for 10-15 minutes, basting and turning the skewers occasionally.
5) Place the vegetable kebabs in the warm serving plates and serve with walnut sauce, if desired.