Pop these back into the fridge for up to eight hours; once you're ready to cook these, pull them out of the fridge about 15 minutes. In order to get a good sear on these, you'll want to dry them off with paper towels before you put them into the preheated (over medium high heat) cast iron skillet.
When you preheat your skillet, put the oil in first, then after a minute or so, add in the butter. This way the butter won't burn, and the oil won't get heated to the point of smoking. If you're using cast iron, it will preheat in just a couple of minutes, so don't walk away from this. Carefully lay your fish in, and cook about 3 minutes per side, until golden brown and delicious!