Prepare a dressing by mixing a splash of olive oil with rosemary, salt, pepper and the juice of 3 lemons.
Cover the cleaned filets with the dressing and let stand for a few minutes.
Preheat the grill and place the filets. The ideal is to only cook them for about 4 or 5 minutes per side, but it will depend on taste. Check with the thermometer as it should have a minimum of 145° F (63° C) internal temperature.