Add the mirin, sake, and sugar to a small saucepan. Heat it over medium and whisk to combine. Add the soy sauce and bring to a boil, reduce to a simmer.
Simmer for about 10-15 minutes or until the sauce reduces to 1/3 of its volume.
When almost ready, the sauce will thicken and you will see more bubbles. Measure the volume with the chopsticks and remove the sauce from the heat.
Cut the unagi fillet in half, thirds, or quarters to fit the donburi serving bowls.
Preheat the broiler on high (550ºF/288ºC) for 5 minutes with the rack placed in the center of the oven (about 8 inches away from the top).
Line a baking tray with aluminum foil for easy cleanup and brush or spray the foil with oil. Put the cut unagi pieces on the foil with the skin side down.
Broil them for about 5-7 minutes until the surface is slightly blistered.
Open the oven and carefully yet generously brush the top of the unagi with the unagi sauce. Then broil for another 30-60 seconds until the sauce is bubbling.
Place the hot steamed rice in individual bowls and brush it with some sauce.
Place fillets of unagi in each rice bowl, then pour or brush more unagi sauce on top. Serve immediately with optional sansho pepper to garnish.