Take the frozen mackerel fillet out of package and soak in the rice starch water for 20 minutes to thaw.
Meanwhile make the sauce by mixing all the ingredients.
Take the fillets out of starch water and pat with paper towel to dry. Coat with corn starch lightly on both sides.
Heat oil in the nonstick surface skillet over medium heat. Sear the fillet, the skin side first, for 3-4 minutes. Flip and sear the other side for 3-4 minutes as well.
Reduce the heat to low, and turn the fillet again so the meaty side is facing you. Spread the sauce over the fillets all around, about 1 Tbsp per fillet. Cover with lid and simmer for 5 minutes so the sauce can set on the fillets.
Place fillets over onions on the platter and serve warm with rice. Enjoy!
