Cut the catfish filet in half. Season both sides of the catfish fillets with salt and black pepper.
Coat the fillets with 1 ½ Tbsp all-purpose flour. Gently shake off excess flour.
Heat 1 ½ Tbsp vegetable oil (if you like crispier texture, use 2 to 2 ½ Tbsp oil) in the frying pan over medium heat. Cook both sides of the fillet until golden brown, about 3-4 minutes each side.
Whisk 2 Tbsp sake, 2 Tbsp mirin, 3 Tbsp soy sauce, and 1 Tbsp sugar together in a small bowl. Pour the sauce over the fish. Using a spoon, scoop some sauce and pour over the fish several times. Cook a few minutes until the sauce gets thicken.
Thinly slice the green onions and sprinkle over the fish. Turn off the heat and serve the fish over steamed rice. Drizzle the sauce over the fish. Sprinkle sesame seeds and sansho pepper if you like.