About two-thirds into your cooking time, pull it out of the oven and press on it to see how well done it is. Feel it. Give the pan a jiggle. You can always use a thermometer; it should be 115 for rare or 135 for medium. But not every piece is the same thickness all the way across, so I also like to test the firmness. If it’s about the firmness of the muscle on the back of your hand, behind your thumb, it’s rare. Squeeze your thumb against your forefinger, that is about medium rare, clench your fist, that is well done.
I always pull the fish out early and test, because you can always put it back in for a minute or two. Over time, you get to know how you like your own fish and learn the “feel” of it, which I think is easier than following recipes. You can always make a sauce on the cooktop as well, however you like it.