Set a large cast-iron skillet over high heat. When a drop of water skitters on the surface, after about 3 minutes, add the oil. Tilt the pan to coat the bottom evenly and heat until the oil is almost smoking, about 30 seconds. Season the salmon with the salt and pepper and add to the skillet, skinned side up. Cook until golden brown on the bottom, about 4 minutes. Turn the salmon, lower the heat to moderate and sear until just cooked, 3 to 4 minutes longer.