Take a small sauce pan and add 3 tbsp turbinado sugar and 1 tsp dark brown sugar. Place the pan on the stove over a medium-high heat.
When the sugar begins to melt, add 75 ml sake and 75 ml mirin. Boil for 1 to 2 minutes, stirring occasionally to evenly distribute the heat.
Lower the heat to a simmer and add 75 ml Japanese soy sauce (koikuchi shoyu). If you want to add extra depth to your sauce, add grilled bones and heads from the eels.
Simmer for about 10 minutes or until thickened to a syrup-like consistency. Stir occasionally to prevent the sugar from burning. Remove from the heat and allow to cool while preparing the eel. (If you added the bones and heads, pour through a strainer and discard.)
Unaju
Wash 300 g filleted eels under cold running water and pat them dry with kitchen paper. Lay a sheet of plastic wrap over a chopping board and place the eel on top. Cut into halves or thirds by gently pressing a sharp knife into the area you want to cut, and then moving the eel back and forth (rather than moving the knife).
Take a large frying pan and place the eel skin side down. Add 2 tbsp sake and place it on the stove.
Place a lid on and steam for 3 minutes over a medium-low heat. While you wait, preheat the grill or broiler on medium-high.
Once 5 minutes are up, remove the pan from the heat and transfer the eel to a wire rack lined with aluminum foil. Place it with the skin side down first and grill for 6 minutes.
Turn over and grill the skin side for 5 minutes or until lightly charred.
Brush the sauce generously over the skin and grill for 30 seconds.
Flip and apply the sauce to the meat side, then grill for 30 seconds. Repeat the brushing and grilling 3 times on each side.
Dish up 2 portions cooked Japanese short-grain rice in a jubako (or similar box) and brush the top with the leftover unagi sauce.
Arrange the eel on top, drizzle with any leftover sauce and sprinkle with Japanese sansho pepper.