Preheat outdoor or indoor grill to MEDIUM-HIGH (or oven to 450 degrees). Place fish fillets on a lightly oiled sheet of aluminum foil wrap (or in a baking dish, if cooking in the oven).
Sprinkle with salt and pepper to taste. In a medium bowl, combine tomato, green onion and basil. Spoon mixture over fish then dot with butter. Lay another sheet of aluminum foil over the top and fold edges together, to make a packet (or cover baking dish).
Grill/bake for 8 to 10 minutes or until fish flakes easily when tested with a fork. Turn off the grill or oven. Remove the top sheet of foil. Sprinkle fish with cheese and return to the grill or oven just to melt the cheese. Serve immediately and enjoy!!