I surmised the original recipe may have used "mountain yam" 山芋 but I only had the domesticated version called "Nagaimo" 長芋. Since nagaimo is not as sticky as yamaimo and a bit more watery, I decided to mix in an egg.
I grated the nagaimo (about 1 cup) and mixed in soy sauce (1 tsp) (#1). I then added one beaten egg and poured it in a non-stick frying pan with a small amount of vegetable oil (1 tbs) on a medium flame (#2). When the bottom was set, I spread finely chopped scallion on top (3 stalks) (#3). Using a spatula, I started folding the edges toward the center all around (#4). After several minutes, I flipped the pancake until both sides were brown (#5). I placed three slices of smoked aged cheddar cheese on top (my wife's choice) and put the lid on for another 1-2 minutes until the cheese melted (#6).
This is a very interesting dish--in a good way. The center was still soft and the scallion flavor was well integrated. It had the mouth feel of the most tender-delicate french omelet you've ever tasted and yet it was very clearly not an omelet. My wife's choice of smoked cheddar added depth of flavor and almost a sense of barbecue that went surprisingly well with the bonito. We really liked this dish and cold sake was the perfect choice.