1. If you can't find mizuna, you can use baby spinach leaves or English spinach (blanched, cut into bite size).
2. I used store bought gomadare for this salad. My favourite gomadare is the brand Mizkan and called “Kin no Gomadare” (金のごまだれ, golden sesame sauce). You can buy it at Asian/Japanese grocery stores. You can get other brands, of course but remember that the flavour of gomadare varies depending on the brand and it is a matter of finding your favourite flavour.
You could use gomadare in my post Japanese Dressings if you like, instead of using a store bought sesame sauce. But the flavour is different as my favourite store bought sesame sauce is creamy and richer with a milder acidity.
3. The boiling time varies depending on freshness and lotus root variety. In Japan, you only need a few minutes to cook it. But whenever I make lotus root salad here in Sydney, I have to cook the sliced lotus root for around 10 minutes. If you like a crunchier texture, I suggest you boil it for less time than 10 minutes.
4. When you add gomadare while the lotus root slices are still warm, the flavour of the gomadare is absorbed into the lotus root. It add to the flavour of the salad and the lotus root slices taste even better.