Add the sake, soy sauce, and mirin to a saucepan over a medium heat without a lid. Once it starts boiling, then add the shaved dried bonito and simmer for 2 minutes.
Turn off the heat. Take out and discard the shaved dried bonito. Let it cool.
Peel the onion, trim the top and root slightly, and make a shallow cross-cut on the bottom. Place them cut-side down in a microwave-safe bowl.
Cover with a microwave-safe plate and microwave at 600W for about 8 minutes, until soft and translucent.
While still hot, drizzle with dashi soy sauce and olive oil, then top with shaved, dried bonito. Enjoy with a spoon.
Notes
💡 Note: dashi soy sauce - keep refrigerated, use within 1 week.