Prepare a hot fire in a grill. Oil the grate and a perforated grill rack.
To make the rub, combine all the ingredients in a small jar with a tight-fitting lid. Secure the lid and shake to blend.
Brush or spray the fillets on both sides with olive oil and sprinkle each fillet with about 1 teaspoon of the rub. (Save the rest for later use; it will keep in the cupboard for several months.)
Place the fish, flesh side down, on the oiled perforated grill rack and grill for 10 minutes per inch of thickness, turning once halfway through. A fish fillet is done when it begins to flake when tested with a fork in the thickest part.
Remove from the grill and season with salt and pepper. Combine the melted butter and lemon juice and drizzle the lemon butter over the grilled fish. Serve hot.