Heat a grill pan over medium-high heat.
Combine first 6 ingredients in a small bowl.
Brush 3 teaspoons oil evenly over onion and bell pepper; sprinkle with 1/4 teaspoon salt. Place onion and bell pepper on grill pan; cook 3 1/2 minutes on each side or until tender. Remove from pan; keep warm.
Brush 2 teaspoons oil evenly over tuna; sprinkle with remaining 1/4 teaspoon salt, cumin, and black pepper. Place tuna on grill pan; cook 3 1/2 minutes on each side or until medium-rare or desired degree of doneness.
Split buns in half horizontally. Spread mayonnaise mixture evenly over cut sides of buns. Top bottom half of each bun with lettuce, 1 bell pepper quarter, 1 onion slice, and 1 tuna steak. Cover with top halves of buns.