Weigh your saba fish and measure 10% of the fish's weight of shio koji (e.g. 200g saba fish, 20g shio koji)
If you have the habit of washing your fish, pat dry the saba fish with paper towels .
Combine the saba fish and shio koji in a ziploc bag. Squeeze the air out of the ziploc bag and ensure the saba fish is coated with the shio koji on both sides. Marinate in the refrigerator overnight.
Preheat oven to 200°C / 400°F on the fan-forced grill mode. If this function is not available, preheat oven to 215°C / 420°F in a regular oven (or 200°C / 400°F in a fan oven).
While the oven preheats, remove the saba fish from the ziploc bag and use paper towel to wipe away the shio koji. Line a parchment paper on a tray and place the saba fish on the parchment paper, flesh-side up. Season lightly with salt. Flip the saba fish so that it is skin-side up. Cut some slits on the skin-side of the saba fish, then rub lightly with oil and season lightly with salt.
Bake for 10 minutes in the upper-third of the oven. If the skin looks like it's burning, shift the saba fish to the lower-third of the oven. Bake for another 5 minutes until the flesh is opaque in colour and flakes away easily.
The 1/2 saba fish that I bought weighs about 200 grams.
Do air the oven by leaving the oven door ajar overnight after cooking so that the fish smell doesn't linger in the oven.
If you have no oven, you can simply air-fry or pan-fry the saba fish.
If you don't like saba fish, you can replace it with whatever fish fillets you like, but the baking times may have to be adjusted, depending on the thickness of the fish fillets.