Preheat a heavy skillet (preferably cast iron) over high heat. Add oil to hot pan. When oil is shimmering hot, carefully slide in half of the fish. Let cook undisturbed until fish lifts easily and top of the fish is turning opaque at the edges, 2-3 minutes. Carefully flip and finish cooking on the other side until fish is just opaque at the centre and flakes easily with a fork, 1-2 minutes more.