In a saucepan over medium-high heat, melt equal parts turbinado and dark brown sugar until fully dissolved. Stir in 2 tablespoons of sake and a splash of mirin, bringing the mixture to a gentle boil for 1-2 minutes. Add a generous pour of Japanese soy sauce, then let the sauce simmer for about 10 minutes until it thickens slightly. Remove from heat and set aside to cool.

Rinse 300 grams of eel fillets under cold water and place them on a cutting board wrapped in plastic to prevent sticking. Using a sharp knife, cut the eel into 2-3 pieces, about 4 inches each.

Arrange the eel pieces in a cold frying pan, skin-side down. Pour in the 2 tablespoons of sake and cover the pan with a lid. Turn the heat to medium-low, and allow the eels to steam for about 3 minutes.

Preheat your grill or broiler to medium-high for approximately 5 minutes.

Using a wire rack lined with aluminum foil, carefully place the steamed eel pieces skin-side down on the grill. Grill for about 6 minutes, then gently flip and grill for another 5 minutes until lightly charred.

Brush the grilled eel generously with the prepared Unagi sauce for a rich, sticky glaze. Grill the eel for an additional 30 seconds; then flip and repeat the basting process multiple times.

Scoop cooked Japanese short-grain rice into warm bowls, brush over the remaining sauce, and place the grilled eel pieces on top. Optionally, sprinkle with sansho pepper before serving.