Peel and slice nagaimo into ½ inch (1.3 cm) thickness. Some of you may feel itchy by touching nagaimo. If you're not sure, wear kitchen gloves to protect your skin. Once you peel, nagaimo gets very slimy, so be careful.
Heat the olive oil in a large frying pan and sprinkle a pinch of kosher salt.
Put the nagaimo slices and saute for 3-4 minutes on medium heat. When it has nice char on the bottom, flip and cook for another 3-4 minutes.
Add the soy sauce. Quickly lift up the frying pan and swirl the nagaimo in the frying pan so that the nagaimo will be evenly coated with soy sauce. Moving around nagaimo prevents from getting burnt. It might splatter a little bit, so be careful. This creates delicious flavors.
When the sauce gets thicken and nagaimo has nice char like this, turn off the heat and serve. Just like in the Midnight Diner episode, I serve this dish with mizuna.