Cut the onion into quarters and break into pieces.
Preheat the grill, setting the burners to medium.
In a bowl, combine all the ingredients, except for the fresh herbs.
Thread the vegetables onto the skewers, alternating colours. Place on the grill and cook for 2 to 3 minutes on each side. Brush with the oil mixture a few times during cooking. Sprinkle with the fresh herbs before serving.
Tips: For authentic Greek slouvaki, thread your ingredients on a vine sprig instead of the usual wooden or metal skewers. You will find them in florists and decorating shops.