1. Preheat grill to medium-high heat (350° to 400°).
2. 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.
3. 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.