To Making the Sauce
Combine mirin and sake in a small saucepan and bring it to a boil to evaporate alcohol.
Add sugar and stir until sugar is completely dissolved. Then add soy sauce and bring it to a boil. Then reduce heat and continue cooking on simmer for 10 mins, or until mixture thickens. Turn off the heat to cool it down.
To Prep the Unagi
Preheat your oven to broil at 290°C for 3 mins. Meanwhile, cut unagi in half to fit inside your serving bowls. Line the baking sheet with aluminium foil and brush oil lightly
Put the baking sheet in the middle rack of your oven, and broil on high for 5-7 mins.
After 7 mins or so, take it out and brush the sauce over.
Continue to broil for another 30-60 secs until you see bubbles on top of unagi.
Serve unagi on top of rice and pour/brush more sauce. Serve immediately.
Tip: Use pre-made/frozen unagi from Asian grocers to shave off half the cooking time!
