The robustly flavored sauce gets its inspiration from olives, tomatoes, and fresh thyme, but is simple to prepare. When the topping is this bold, go with a simple grilled technique for the fish.
Preheat grill to medium-high heat (350° to 400°).
Heat olive oil in a large skillet over medium heat. Add onion, and cook, stirring occasionally, 5 to 7 minutes or until tender and lightly browned. Add tomatoes and olives, and cook, stirring, 3 to 4 minutes or until mixture is well blended and tomato juice has reduced slightly. Remove from heat, and stir in thyme and 1/2 teaspoon salt.
Sprinkle tuna steaks with salt and pepper. Grill on greased grill racks 2 minutes on each side or until desired degree of doneness. Transfer tuna steaks to individual plates, and top evenly with tomato-olive mixture.