Once the walnuts are toasted and skinned, put them in a mortar and pestle, or better yet a food processor with chopping blade, and crush them up as finely as you prefer. I like it to be quite fine with a few small chunks. Add the mirin, miso and sugar and stir well until combined. (If you can’t find or can’t use mirin, either use the same amount of sake with a pinch more sugar, or just leave it out.)