Mince the onion, garlic, and ginger in a food processor (You can grate them as well).
Add the sake and mirin into a pot. Turn on the heat, and evaporate the alcohol. Then add all the remaining ingredients.
When it starts to boil, turn the heat down to very low, and simmer for 2 minutes. Leave it to cool. It tastes better if you let it rest for more than 2-3 hours.
It's also perfect with cubed cuts of steaks!