Bring the grill up to 375 degrees on direct heat. As we'll be using the cast iron skillet, it will create an evenly heated surface to cook the fish on.
Add a couple of tablespoons of olive oil to the skillet.
Sprinkle a little bit of salt on the fillet and lay it in the cast iron. Let it cook for three minutes and turn it over.
Pour melted butter into the skillet. It will quickly start to bubble. Use a tablespoon to scoop the butter over the fish. You will see the butter start to brown and take on a nutty flavor.
After another three to four minutes, the fish will be thoroughly cooked and the butter will be browned. Sprinkle the fresh herbs on the fillet and drizzle it with lemon juice.
Make sure the internal temperature has reached 145 degrees. I used a wireless meat thermometer to verify.
Don't Rush: Don't rush the heating process. The skillet must come fully up to temperature before you add the fish. I always add a drop of water to make sure it is sizzling right away.Cast Iron Caution: Be careful with cast iron on the grill. While the benefit is that it retains heat well, which makes it perfect for grilling the fish, it also means that it stays hot a long time after you remove it. Trust me, it can burn your hands easily.Brown Butter: Keep ladling the butter over the fish. It will separate and within about three minutes, you'll see the butter darken substantially. That will create an intense flavor.Melted Butter for Brown Butter: This fish cooks so fast that I decided to pre-melt the butter but allow it to "brown" in the cast iron. If you've ever made brown butter, you know that you melt and heat up butter until it starts to smell a little nutty. It is right before it smells like it does for caramel corn. Stir and scoop butter over the fish, allow it to drizzle off and do that 10-15 times. You can smell the spoon to check when the butter browns. It's a good way to test it.