Remove the stalks from the shiitake mushrooms and peel away the skins. Use a sharp knife to slice the shiitake horizontally across the cap, into several thin slices; ideally, no more than 2 mm (1/16 inch) thick. Spread the freshly sliced shiitake mushrooms out on a large tray. Evenly sprinkle the sea salt over the mushroom slices. Pour the grapeseed oil over and turn to ensure the slices are well coated. Put into a cryovac bag and seal using a vacuum machine on medium pressure. Keep the mushrooms at room temperature in the cryovac bag for 8–12 hours. This will intensify the flavour of the shiitake mushrooms.