MARINATE onions, peppers and cucumbers in vinegar.
MEANWHILE, coat tuna with Cajun seasoning. Grill 3 to 4 minutes on each side or until tuna flakes easily with fork.
MIX dressing and garlic powder. Drain vegetables; discard marinade.
FOR each serving: Place 3/4 cup of the vegetable mixture on each serving plate. Top with 2 (3-oz.) tuna pieces; drizzle with 1 Tbsp. of the dressing mixture.