Sauté sliced onions in hot olive oil over medium-high heat 8 minutes or until tender. Add chopped tomatoes, next 4 ingredients, and 1/4 teaspoon each of salt and pepper. Simmer, stirring occasionally, 5 minutes. Set onion-and-tomato mixture aside.
Place fish in a single layer on a lightly greased rack in an aluminum foil-lined broiler pan; sprinkle with remaining 1/4 teaspoon each of salt and pepper.
Broil 5 inches from heat 8 minutes or until fish flakes with a fork.
Spoon onion-and-tomato mixture evenly onto a platter; top with fish fillets. Sprinkle evenly with crumbled feta cheese, and garnish, if desired.