Thaw fish, if frozen. Rinse fish and pat dry with paper towels. Measure thickness of fish; set aside.
In a 10-inch skillet, combine chicken broth, onion, oregano, lemon peel, lemon juice, and garlic. Bring mixture to boiling; reduce heat. Cover and simmer about 3 minutes or until onion is tender.
Arrange fish fillets in onion mixture. Add peppers. Cover and simmer over medium heat 4 to 6 minutes for each 1/2-inch thickness of fish or until fish flakes easily when tested with a fork. Use a slotted spoon to transfer fish and vegetable mixture to a serving platter. Keep warm.
Combine cornstarch and water. Add to pan juices. Cook and stir until thickened and bubbly. Cook and stir 2 minutes more. Spoon atop fish and vegetables. Garnish with lemon. Makes 4 servings.