1 sm. onion, sliced
2 tbsp. butter
1 lb. flounder or sole fillets
2 tbsp. lemon juice
2 tsp. dried oregano
2 c. (about 1 lb.) sliced zucchini
1 c. fresh mushrooms
1/2 c. chopped tomatoes
1/2 c. shredded Swiss cheese

In a large skillet cook onion in butter until tender, about 5 minutes. Add fish, lemon juice, oregano, zucchini and mushrooms. Cover and simmer 10 minutes. Top with tomato and cheese. Simmer 2 minutes longer or until cheese melts.