DO-AHEAD TIP: Refrigerate flounder for 30 minutes (see recipe).
Preheat oven to 375 degrees.
In a small bowl, combine first 5 ingredients (mustard through black pepper); blend well then rub mixture over the top side of each fillet; cover and refrigerate for 30 minutes.
Place flounder fillets, plain sides down, in a 9- x 13-inch baking dish that has been lightly coated with olive oil; bake for 20 to 25 minutes or until fish flakes easily when tested with a fork.