In a charcoal grill, gas grill or broiler, prepare a fire.
Away from the heat source, lightly coat the grill rack or broiler pan with cooking spray. Position the cooking rack 4 to 6 inches from the heat source.
Sprinkle the tuna fillets with pepper. Place the fillets on the grill rack or broiler pan. Brush the tuna with 2 tablespoons of the Caesar dressing while cooking.
Grill or broil until the fish is opaque throughout when tested with the tip of a knife, about 8 minutes.
Just before taking the tuna off the grill, place buns on grill or broiler pan to toast.
Place the tuna steaks on the buns. Top with lettuce and tomato. Drizzle with the remaining 2 tablespoons of Caesar dressing.
Serve immediately.