1. If dried bonito flakes are not your preferred topping, you could substitute roasted white sesame. The flavour is quite different of course but it looks as good.
2. Boiling time varies depending on the type of spinach you can get and also depending on your preference for doneness. Some people prefer al-dente, some like well-cooked.
3. The root end of spinach is pink and it is edible. It's actually very sweet and I must say it is the best part of the spinach. Unfortunately, this time round, the root end of the spinach I bought was quite ugly and not nice at all. Hence you cannot see the pink part of the spinach in the photos. If your spinach root is clean and soft, include it when cutting the spinach.
4. Instead of soy sauce, you could serve with ponzu which is a citrus-based soy dressing. Please refer to Japanese Dressings for the recipes.