Whisk together soy sauce, vinegar, pepper, Sambal Oelek or chili sauce and water and set aside.
Heat olive oil in sauté pan on medium then add onion and sauté until flavor releases.
Carefully place Spanish mackerel fillets, skin down in pan, pushing them in between the onion slices.
Pour the soy sauce mixture over the fish and onions.
Allow the sauce to heat through and bubble, shaking the pan occasionally to stop the mackerel from sticking to the pan.
Once sauce is bubbling, lower heat to simmer and cover.
After three minutes, remove the cover, and baste the fish with the sauce then replace cover.
Repeat, then remove from heat once fillets become opaque and begin to flake.
Serve with your favorite side dishes.