Turn on the kitchen fan — a proper sear creates smoke. Heat the grapeseed oil in a large 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 mushrooms — they should sizzle immediately when they hit the pan. Be careful not to overcrowd the pan; you may need to sear them in batches depending on your pan size. Toss them gently with the grapeseed oil and sprinkle evenly with the salt. Turn the mushrooms so that the cut side is facing down. Lower the heat to between medium and medium-high.
Leave the mushrooms undisturbed for 3 to 5 minutes. Take a peek — the side facing down should be evenly golden brown. When it is, move the mushrooms around, stirring them gently to allow the other sides to brown as well.