Cut each onion quarter into thirds. In a medium bowl combine marinade, tuna, bell pepper and onion pieces; toss. Cover and refrigerate 30 to 45 minutes.
Remove tuna and vegetables from marinade; discard marinade. Thread 4 skewers with bell pepper, onion, tuna, tomato and tuna again, repeat twice, ending with a tomato.
Preheat boiler. Lightly coat baking sheet with nonstick cooking pray. Place skewer on baking sheet. Broil 2 to 3 minutes per side. Transfer skewers to a serving platter and serve immediately.