Place the first 7 ingredients into a medium bowl and mix to combine. Divide and shape the mixture into 16 oval balls. Thread 4 meatballs (lengthwise), 4 green bell pepper cubes and 4 red onion wedges onto each of the skewers. Preheat your barbecue to medium. Brush the meat and vegetables with the olive oil. Lightly oil the grill if prone to sticking. Grill the kebabs 4 to 5 minutes per side, or until the meat is entirely cooked through. Serve the kebabs with the tzatziki sauce for dipping.