A flounder fillet is a healthy, high-protein, low-calorie, fast-cooking, light, delicately flavored piece of fish to which you can do just about anything your palate desires. These particularly thin fillets are tricky to cook on the grill or on the stove, but they're entirely manageable in the oven. Because fish is naturally high in healthy fats and moisture, they do well baked or broiled. However, with even just a minute or two too long in the oven, flounder begins rapidly drying out, so pay close attention to your meal when preparing it.
Preheat your oven to 425 degrees Fahrenheit about 30 minutes ahead of time if you're baking the flounder fillet. If you want to broil it, turn the broiler function on about 10 minutes in advance to preheat. Considering preheating and cooking time, broiling takes less than half the time of baking; it's also better at crisping the outside of fish, so opt for broiling if you're breading the fillet or prefer a crusted exterior.
Line a baking tray with foil and apply a coating of cooking oil to bake the flounder, or grease the broiler rack with cooking oil. Don't skip greasing, as it prevents sticking; if a thin, delicate, flaky cooked flounder fillet sticks, there's considerable risk of it falling apart when you try to get it off the tray.
Drizzle cooking oil or melted unsalted butter over the flounder. Pat on salt and pepper liberally, along with any other aromatics, herbs, spices or flavoring agents you'd like to use. Use blackening or Cajun seasoning mixes for spicy preparations or some combination of dill, basil, tarragon, thyme, oregano, marjoram and rosemary; press the top of the fillet in panko breadcrumbs mixed with herbs, finely chopped pecans or pistachios, grated Parmesan cheese or a citrus zest; use minced garlic or ginger; or brush the top with honey mustard, Dijon mustard or minced horseradish.
Put the fillet on the lined baking tray or the broiler pan. To bake, put the fish into the middle of the oven; to broil, position the surface of the fillet 4 to 5 inches below the heating element. Bake a flounder fillet for about 12 minutes or broil it for about 6 minutes. It's done as soon as the flesh flakes apart easily and is uniformly opaque inside. Remove the fish from the tray or pan carefully with a broad spatula, supporting it fully underneath.