How to reuse leftover bonito flakes: Spread it out on parchment paper-lined baking sheet. Bake in an oven preheated to 195-210°F/90-100°C. Around the time you've forgotten about it, it will be nicely dried (about 1 hour). P.S. Be careful not heat the oven too high, this will dry out the flakes. You can also dry-roast in a frying pan.