Thaw fish, if frozen. Rinse fish; pat dry with paper towels. Set aside. For sauce: In a small saucepan, heat oil over medium heat. Add onion and garlic; cook about 5 minutes or until tender, stirring occasionally. Add tomatoes, the snipped or dried thyme, olives, and capers. Heat to boiling; reduce heat to medium. Simmer, uncovered, about 10 minutes or until most of the liquid has evaporated.

Meanwhile, preheat broiler. Measure thickness of fish. Place fish on the greased unheated rack of a broiler pan, tucking under any thin edges. Broil 3 to 4 inches from heat for 4 to 6 minutes per 1/2-inch thickness of fish or until fish flakes easily when tested with a fork, turning fish once if fillets are 1 inch thick. Serve with sauce. If desired, garnish with fresh thyme sprigs. Makes 4 servings.