Heat a grill plate or frying pan over medium-high heat. Heat half of the olive oil. Season the fish fillets with salt and pepper and cook for 4 minutes per side or until cooked through and slightly golden on the outside. Remove from the pan and set aside to rest.
Meanwhile, combine the onion, tomatoes, basil, balsamic vinegar and remaining olive oil in a small bowl. Toss to combine.
To serve place one fillet of fish on a dinner plate, top with 3-4 tbsp of salsa and a small sprinkle of feta cheese. Serve and enjoy!