Unadon (Eel Rice)

2 unagi (eel) fillets, about 160 g each
red pepper, optional
UNAGI SAUCE for 2 fillets:--
¼ cup soy sauce
¼ cup mirin
2½ tbsp sugar
1½ tbsp saké
UNAGI SAUCE:--
¾ cup soy sauce
¾ cup mirin
½ cup sugar
¼ cup saké

UNAGI SAUCE
In a small saucepan, add mirin, saké, sugar.
Turn on the heat to medium heat and whisk all together. Add soy sauce and bring to a boil.
Once boiling, reduce heat to a gentle simmer (you should see bubbles around the edges) and continue simmering for 10-15 minutes. (or 20 minutes if you're making the full amount) The liquid should reduce to roughly ⅓ of the original amount. (The mark on a chopstick inserted in the sauce will give you a rough estimate)
Toward the end of cooking, you will see more bubbles. When the sauce thickens, (check with the chopstick again to see if the sauce is ⅓ of the original amount) remove from the heat. As it cools, the sauce will thicken more. If you feel you've reduced too much, add a bit of water and cook slightly more to the right consistency. You can store the sauce in an airtight jar and keep in the refrigerator for up to 3 months.
TO GRILL
Preheat the oven grill to HIGH (290ºC) with a rack placed about 20 cm away from the top heating element (in the centre of the oven) for 5 minutes. (When broiling, you don't control the temperature in the oven; instead, you control the distance between the broiler and the surface of the food. It's similar to using hotter and cooler zones on your grill.)
Line a baking tray with foil for easy cleaning and brush/spray the foil with oil. Cut the unagi in half. (or thirds, depending on the serving bowl size to fit the unagi fillet)
Place it on the foil, skin side down. Broil the unagi until the surface is blistered a bit, about 5-7 minutes. No need to flip.
Open the oven and brush the unagi with the sauce. Grill again for 30-60 seconds until you see the sauce bubbling on top of unagi.
TO BAKE
Preheat the oven to 220ºC with a rack placed in the middle and bake the unagi on baking paper until the surface is blistered a bit, about 10-12 minutes. No need to flip. Brush the unagi with the sauce and bake again for 30-60 seconds until you see the sauce bubbling on top of unagi.
TO PAN FRY
Wrap the unagi in foil and reheat on low heat for 5-8 minutes. You won't get nice blisters/chars if you use this method.
TO SERVE
Serve unagi on top of rice and pour/brush more unagi sauce. You can also sprinkle crushed red pepper at the table. Serve immediately.
To serve in the jubako (Japanese lacquered box), it looks nice to serve 2 pieces (the entire fillet), overlapping each other.
TO STORE
Keep the leftovers in an airtight container and store in the refrigerator for 3 days or in the freezer for 2 weeks.
Note: Grill settings are LOW (230ºC), MEDIUM (260ºC), and HIGH (290ºC).