Add 3 tbsp turbinado sugar and 1 tsp dark brown sugar to a saucepan. Place the pan on the stove and turn on the heat to medium-high.
Once the sugar starts to melt, add 5 tbsp sake and 5 tbsp mirin. Bring to boil and allow to cook for 1-2 minutes, stirring occasionally.
Add 5 tbsp Japanese soy sauce (koikuchi shoyu) and turn down the heat to simmer.
Simmer until thickened slightly (about 10 minutes). Stir occasionally to prevent the sugar from burning. Remove from the heat and allow to cool while preparing the eel.
Unagi don
Wash 300 g filleted eels with cold running water. Cover a chopping board with a sheet of plastic wrap and place the eel on top. Cut each eel into 2-3 pieces by pressing a sharp knife into the area you want to cut, and pushing the eel back and forth over the plastic wrap.
Place the eel in a cold frying pan with the skin side down and add 2 tbsp sake.
Turn on the heat on medium-low and steam for 3 minutes with the lid on. While it's steaming, preheat the grill/broiler on medium-high for 5 minutes.
After 5 minutes, remove the pan from the heat and transfer the eel skin side down to a wire rack lined with aluminum foil. Grill for 6 minutes.
Turn the eel over and grill the skin side for 5 minutes or until lightly charred.
Apply the sauce generously but evenly over the skin and grill for 30 seconds.
Turn the eel over and apply the sauce on the meat side and grill for 30 seconds. Repeat brushing and grilling each side twice more (3 times on each side).
Dish up 2 portions cooked Japanese short-grain rice and brush the top with the leftover sauce.
Place the eel on top and sprinkle with Japanese sansho pepper.