Season the fish on both sides. Heat 2 nonstick pans if you have them and put half the sunflower or vegetable oil into each. Add the fish, skin side down. When you put the fish in, it will arch upward from the first contact with the heat. Leave it for a minute or so to warm through, then slowly press the flesh down, from the tail end upward, so that all the skin is in contact with the pan and crisps up evenly. You will see some excess fat coming out of the fish between the flesh and the skin — so blot this up with some paper towels to help the skin to crisp up even more.