Mix all the spice rub ingredients together and remove the flounder from the refrigerator 30 minutes before you plan to put them on the grill.
Cover all surfaces of the steaks with a light to medium covering of the spice rub and gently rub into the flounder. Let this rest in a container outside of the refrigerator to allow it to reach room temperature before putting it on the grill.
Get your grill going and achieve a medium hot grill. Place the flounder filets in a grill basket or on a grill screen with smaller holes to keep the filet from falling apart and crumbling through the grill as it gets closer to done. Cook for approximately 5 to 6 minutes per side. You only need to flip it once on the grill.
If you are unsure of whether it is fully cooked or not, you can test it with a digital meat thermometer at the center of the thickest part of the filet. The temperature should read approximately 135° F.