My mom always made her baked flounder like this, as was typical in "Down East" Carteret County, NC. You can use either whole flounder or flounder fillet. I prefer using the fillet so you don't have to worry about bones. Lining your baking dish with foil makes for an easy clean up.
Preheat oven to 400 degrees. Spray a 9" X 13" baking dish with cooking spray. Place prepared flounder in baking dish with the flesh side up.
Slice peeled potatoes into 1/4-inch thick round slices. Place potato slices around sides of baking dish and around flounder in bottom of pan. Add salt and pepper to taste on fish and potatoes. (This is good with a little extra pepper!) Top with sliced onion rings. Add enough water in the bottom of the baking dish to come up around sides of fish a little, but not to cover it. Dot with butter or margarine on top of fish.
Bake in preheated oven for approximately 45 minutes or until potatoes are tender and flounder flakes with a fork. Enjoy!