How to Make It
In a small bowl, combine the onion, apple, dill, mustard, olive oil, salt and pepper. Place the bluefish fillet, on a large plate and evenly coat both sides of the fish with the onion-apple mixture. Cover with plastic wrap and refrigerate overnight.
In a large skillet, melt the butter over high heat. Add the coated bluefish fillets and cook until the bottoms brown and a crust forms on the first side, about 3 minutes. Using 2 spatulas turn the fish, reduce the heat to moderately high and cook until browned on the second side, about 2 minutes. Remove the fish to a serving platter.
Add the vinegar to the skillet and scrape up any browned bits from the bottom of the pan. Cook, stirring, for 30 seconds. Pour the pan drippings over the fish and serve hot.