In a skillet cook onion in oil over moderately low heat, stirring, until it is softened. Add garlic and cook, stirring, 1 minute. Add wine and boil the mixture until almost all the liquid is evaporated. Add the thyme, oregano and tomatoes with reserved liquid, bring mixture to a boil, and cook it over moderate heat, stirring occasionally, 5 minutes, or until it is thickened slightly. Spread half the sauce in the bottom of a gratin dish just large enough to hold the fish in one layer. Top it with the perch, skin sides down and spread remaining sauce over perch. Sprinkle olives and Feta evenly over the sauce and bake 15 to 20 minutes at 400 degrees F. or until fish just flakes when tested with a fork.