Heat the grill up to 375°F on direct heat. Place the cast iron flat top griddle on the grate.
The cast iron is ready when you can add a drop of water on the surface and it immediately started sizzling.
Add the butter and olive oil to the cast iron and use a spatula to cover the surface.
Add the lingcod fillets and let it cook for 3 - 4 minutes.
They are ready to turn over if they are easy to lift off the cast iron with a spatula. That shows that they have been properly seared.
Turn them over and cook the other side for 3 - 4 minutes.
Remove from the grill and serve with slices of lemon.
Notes
Fully Heat Up: Do not try to put the fish on the cast iron unless it is preheated. You want that initial heat to sear the fish.Light Seasoning: This is a light and sweet flavored fish and tasted better without any seasonings. If you do add some, add it sparingly.Cut Small Sizes: Cut the fillets into manageable sizes before putting them on the grill. It makes turning them over much easier and they will cook faster.Oil Choices: Olive oil typically works at this temperature, but avocado oil will provide a higher smoke point. Steer away from using butter, as the cast iron skillet will be hotter than the smoke point of butter and you risk burning it. Substitute: If you can't find lingcod, try a nice medium sized halibut fillet. The flavors are similar. The halibut may be a bit thicker and will require a few extra minutes on the skillet to be fully cooked.