Preheat grill to medium. (No grill? See Oven Variation, below.)

Combine tomatoes, squash, onion, green beans, olives, lemon juice, oregano, oil, capers, 1/4 teaspoon salt and 1/4 teaspoon pepper in a large bowl.

To make a packet, lay two 20-inch sheets of foil on top of each other (the double layers will help protect the contents from burning); generously coat the top piece with cooking spray. Place one portion of tilapia in the center of the foil. Sprinkle with some of the remaining 1/4 teaspoon salt and pepper, then top with about 3/4 cup of the vegetable mixture.

Bring the short ends of the foil together, leaving enough room in the packet for steam to gather and cook the food. Fold the foil over and pinch to seal. Pinch seams together along the sides. Make sure all the seams are tightly sealed to keep steam from escaping. Repeat with more foil, cooking spray and the remaining fish, salt, pepper and vegetables.

Grill the packets until the fish is cooked through and the vegetables are just tender, about 5 minutes. To serve, carefully open both ends of the packets and allow the steam to escape. Use a spatula to slide the contents onto plates. Oven Variation: Preheat oven to 425 degrees F. Place green beans in a microwavable bowl with 1 tablespoon water. Cover and microwave on High until the beans are just beginning to cook, about 30 seconds. Drain and add to the other vegetables (Step 2). Assemble packets (Steps 3-4). Bake the packets directly on an oven rack until the tilapia is cooked through and the vegetables are just tender, about 20 minutes.