The general rule is to bake flounder for six to 10 minutes per inch of thickness, according to Sea Best, a company that processes frozen flounder filets. The precise cooking time depends both on the filet's thickness and its baking temperature.
A simply prepared flounder filet, unbreaded but sprinkled with butter and lemon juice, bakes for 20 to 30 minutes at 325 F, according to About.com. Alternatively, a Food & Wine recipe for flounder coated with breadcrumbs and Parmesan cheese calls for 15 minutes of baking time in a 425 F oven. Sea Best recommends thawing frozen flounder before preparation and preheating the oven before baking the fish.
Whereas breading and lemon-butter sauce are two common preparations, flounder's flaky texture and mild flavor make it an excellent foundation for a range of dishes, according to the Huffington Post. It can be braised in the oven in Italian or Cajun-inspired tomato sauce, for example, or breaded and quickly sautéed, sauced and then finished in the oven. Breaded flounder offers a lot of taste on its own, but it's also an excellent addition to po'boys or fish tacos.
The Huffington Post notes that the name "flounder" actually applies to a number of varieties of flatfish, but these varieties' flavors and textures are so similar that a single name is appropriate.