Place the pan on the grill and light the grill to high heat. We need to let that pan heat up on the grill for at least 10 minutes, as hot as you can get it.
Rinse and dry the grouper fillets very well. I mean really dry, so that the butter and rub sticks to the fillets. Don’t get lazy on me, use multiple paper towels.
Mix all of the rub ingredients together well and then spread the rub out on a plate.
Melt the butter in a separate shallow dish, large enough to fit the fillets.
Dip each fillet in the butter, covering both sides and then transfer to the plate with rub and coat each side of the fish with the blackening rub.
Immediately place the fish onto the very hot pan on the grill.
Cook, with the lid open, for 3-5 minutes, lifting the bottom of the fish carefully with tongs to check on the crust. We want to create a nice, blackened crust without burning. If you were patient enough to get the grill pan nice and hot, this should only take around 3 minutes.
Flip the blackened grouper and cook for another 3-5 minutes on the other side, creating that great blackened crust (without burning!). The fish should take about 8 to 10 minutes total for an inch thick filet. Adjust based on the thickness. Cook a bit longer if needed to get the fish opaque and starting to get flakey. 145°F or 218°C is what USDA recommends for fish.
Transfer to a plate and serve!