Heat the grill to medium-high. Use grilling tongs to dip a wad of paper towels in some vegetable oil, and use the oil-dipped paper towels to wipe down the grill rack.
Brush the fish on both sides with oil and season with salt and pepper. Place the fish, flesh side down on the hot oiled grill and let cook for 4 to 5 minutes without moving. Use a large, flat, metal spatula, preferably a fish spatula, to gently lift the edge of the fish from the grill. If you see nice grill marks and the fish releases easily from the grill, it is ready to flip. Carefully flip the fish to another clean, oiled part of the grill (you can also give the skin of the fish another generous brush of oil to prevent sticking. Cook on the second side for another 4 to 5 minutes, until the skin is crispy and has nice grill marks and releases easily from the grill.
Carefully use the spatula to transfer the fish to a serving platter. Give the finish a final drizzle of olive oil, and serve with the persillade, if desired.
Notes
For thinner fish steaks, 1 inch or so, you should flip them just once so the inside doesn’t overcook before the exterior browns. With thicker pieces of halibut, however, you can flip them a total of three times to get those nice cross-hatched grill marks. Just turn the steaks 90 degrees as you flip them each time.It’s more important to not allow the fish to dry out than to get nutty about perfect grill marks.