Combine the mirin, sake, and sugar in a small saucepan over medium heat. Whisk together, then add the soy sauce and bring to a boil.

Reduce heat to a gentle simmer and cook until the sauce reduces to roughly one-third of its original volume, about 10-15 minutes. It will thicken further as it cools.

Cut the defrosted eel fillets in half or thirds to fit your serving bowls. Preheat the broiler on high (550°F) for 5 minutes with the rack about 8 inches from the heat.

Line a baking sheet with foil and brush with oil. Place the eel skin-side down and broil for 5-7 minutes until the surface blisters.

Brush the tare sauce generously onto the eel. Broil for another 30-60 seconds until the sauce bubbles and caramelizes.

Serve steamed rice in bowls, brush some tare sauce over the rice, and place the glazed eel on top. Drizzle with more sauce and sprinkle with sansho pepper if desired.