To Make Sauce
Pour mirin and sake into a saucepan and bring to boil. Add sugar and stir until dissolved.
Add soy sauce and bring to boil. Lower heat to simmer for 10 mins or until thickened. Remove from heat and let cool.
To Grill Eel
Preheat oven to 290°C on broil. Place eel on a foil-lined baking tray and insert into the middle rack of oven. Broil for 5-7 mins.
Remove and brush the sauce over the eel. Broil for ½-1 min.
Remove from oven and transfer to a serving plate. Top with more sauce and sprinkle some furikake to garnish. Serve with rice.