Turn on the kitchen fan — a proper sear creates smoke. Season the fish on all sides with the salt. Heat the grapeseed oil in a medium cast-iron skillet until almost smoking. Heat is very important here; the oil will make legs or waves across the pan when it’s ready.
Once the pan is hot, add the fish, skin-side down. The fish should sizzle immediately when it hits the pan. Turn the heat down to between medium and medium-high.
Gently press the fish down into the pan with a spatula for a couple of seconds to ensure that the skin does not curl. Leave the fish undisturbed for 2 to 4 minutes. Take a peek -- the side facing down should be evenly golden brown. Flip it and continue cooking on the other side for another 2 to 4 minutes. The fish should look firm and opaque and come apart with a fork when finished. The internal temperature should also reach 145°F before pulling it from the pan.