Place yam in a sturdy plastic bag and crush it finely using a rolling pin. Add minced pork, mentaiko, sake, and salt and pepper to the bag. Knead until sticky, then cut a 1cm opening at the tip of the bag.

Heat half of the salad oil (1/2 tbsp) in a frying pan and spread thinly. Arrange half of the gyoza wrappers (10 pieces) in a circular pattern. Pipe the filling onto the wrappers and tear the cheese slices to place on top. Cover with the remaining gyoza wrappers (10 pieces).

Cook over medium heat until browned for about 1 minute. Add the flour-water mixture around the edges, cover with a lid, and steam for about 2 minutes. Remove the lid and evaporate excess moisture.