Cut the onion into pieces approximately 1 1/2 inch square. Set aside 24 pieces for the kebabs and reserve any extra onion for another use.
Cut the bottom and top off of each pepper and clean out and discard the seeds. Cut each pepper into 8 rectangular pieces.
Thread the vegetables and kielbasa onto 4 12-inch skewers, inserting peppers, onions, and tomatoes between pieces of kielbasa. Lightly brush with olive oil.
Reduce the grill heat to low and grill kebabs for 5 minutes per side to soften the vegetables and heat the kielbasa through. Garnish with chopped parsley before serving if desired.