Correct temperature and technique for frying in cast iron I recently bought a cast-iron grill pan, in hopes to recreate the great taste of grilled salmon steaks I ate recently at a restaurant.

Before trying it, I had received advice that I should let the salmon stay in the pan until it was released from the pan by itself, instead of trying to turn it early. Then, according to the advice, it wouldn’t stick.

First attempt went bad. The fish stuck to the pan, I barely managed to turn it, in gazillion pieces. The opposite side of the steaks, after turning it, didn’t stick at all. Obviously, this might be because of all the extra fat in the pan after frying the first side.

By salmon steaks I mean what you see in the first picture here, that is, the flesh and not the skin had direct contact with the pan:

https://www.chabad.org/recipes/recipe_cdo/aid/4520/jewish/Salmon-Steaks.htm

Next time I tried pork cutlets (from the neck). In the meantime, I had found more advice, namely that I shouldn’t put the food into the pan before it was hot, but I didn’t know how hot it had to be.

This time, it worked exactly as the first advice had predicted: At first, the cutlets stuck immediately to the pan, in a tiny fraction of a second, like it had been glued with superglue. Then, when the time came to turn it, it wasn’t even the least bit stuck! It was completely loose, I could have easily turned it with one finger, without any residue, if it hadn’t been hot. Just like non-stick. But this time it was the other way around, after turning them, they stuck and when they were past done, they were still partly stuck. Despite way more fat in the pan, from frying the first side.

So, I decided to season the skillet. For three days I seasoned it according to the best advice I could find. Surface now dark, smooth and shiny.

So, I tried again with trout steaks this time. I used exactly the same oiling technique and amount as with the pork cutlets, and tried to replicate the frying technique w.r.t. heat and when to put it in the pan. This time it was glued to the pan when the time came to turn it.

So, this is not a question about seasoning, because there’s tons of advice about that on the internet. But the initial success with the pork cutlets was in a completely new unseasoned pan.

The question is:

What is the correct frying technique, as in, how do I know how hot it should be when I put in the food, does it matter w.r.t. sticking, basically, what are the details to achieve that effect that the food suddenly becomes released from the pan, that I had with the pork cutlets?

Finally, could it be a good idea to get one of the IR-thermometers, that measure the temp of the pan without touching it, to achieve the same level of control as with an oven?
There are a few things to keep in mind when cooking with cast iron:


It must be seasoned properly. The biggest mistake people make here is to have the oil too thick when seasoning. You need to wipe the cast iron with a dry paper towel to remove as much oil as possible. If your pan feels sticky when clean you've used too much oil and should start over.
Yes, you want your pan hot before adding fish (see below). 
You want your pan evenly heated. Cast iron is a very poor conductor of heat and it takes a while to fully heat evenly. You're best heating it a lower flame/setting for a longer period before putting in your food.
A little oil rubbed on both sides of your food will help. 
The advice you got about letting the fish release is 100% correct. That's a common mistake people make. As you found, it works like magic. Some things, like eggs, will always stick and are best for a non-stick pan rather than cast iron (even really well-seasoned cast iron). 
In terms of temperature, I would suggest maybe 400-425° for the flesh side and 450° for the skin side. I generally cook the fish first on the flesh side long enough to leave the perfect color marks and then cook mostly on the skin side. If you score the skin it will be crispier (and salt also helps). A temp gun is really useful, and I highly encourage you to purchase one, but flicking some water on the pan will also give you a pretty good indication of how hot it is. If your water evaporates slowly it's too cold. If it evaporates quickly then the pan is hotter, but not hot enough. If it rolls around the pan and takes some time to evaporate, then your pan is hotter still (at least 380°F), and probably about the range you want. The reason it takes longer to evaporate when really hot is that steam under the water droplets is insulating the droplet from the pan, causing it to take longer to evaporate. (This is called the Leidenfrost effect.)