2. Lay a large piece of tin foil on a work surface. Place the fish fillets in a single layer in the center of the foil and season them with salt and pepper.
3. Arrange the onion and tomato slices on top of the fish. Sprinkle the basil on top. Arrange the squash and red pepper on top of the onion and tomato. Season with salt and pepper and drizzle the olive oil over the vegetables.
4. Pull the sides of the foil up around the fish and crimp them shut to enclose the fish. Place the foil packet on the grill and cook for 10 to 12 minutes until the fish is cooked through.
5. Carefully open the foil pouch. Transfer the fillets and vegetables to serving plates or a platter and serve.