Heat the oil in a 12-inch cast-iron skillet or nonstick pan over medium-low heat. Season the fish well with salt and pepper. Cook, skin side down and undisturbed, for 7 minutes. The fillets will cook from the bottom up so that the flesh stays moist while the skin gets so crisp that it crackles; if the skin becomes crisp before the top of the fish finishes cooking (it should look just opaque), flip it over and cook very briefly to finish. Serve with a lemon wedge.