Liberally season the flesh side of the mackerel with the Creole seasoning.
Heat 2 tablespoons of the olive oil in a 12-inch fry pan over medium-high heat. Add the mackerel, seasoned side down. Season the skin with the salt and cook for 3 minutes. Flip the fish to the other side and add the remaining tablespoon of olive oil, the yellow onion, parsley, and orange juice. Turn off the heat and let sit for 3 minutes.
Serve the fish hot with crusty bread.