Preheat the grill. Take 2 large pieces of foil and cross them. Drizzle olive oil, salt and half of the basil leaves on the foil. Place haddock fillets on top.
Add tomatoes, remaining olive oil, salt and basil leaves, then squeeze the lemon juice over the fish.
Close up the foil and fold into a packet. Add another piece of foil if needed to make sure the fish is completely enclosed.
Place on direct heat on the grill and close grill. After 10 minutes, move to indirect heat. Close grill and continue to cook for another 20 to 25 minutes.
Take off the grill and serve! Sprinkle fish with feta if you like.