Pick the leaves from the mitsuba. Blanch in boiling water for 30 seconds, then refresh in iced water. Squeeze out excess water. Place the leaves onto a dehydrator tray in a food dehydrator and dehydrate at 55°C (130°F) for 24 hours or until fully dehydrated.
Place the dried mitsuba leaves in a spice grinder and grind into a fine powder. Pass the powder through a fine chinois and store i