Preheat your grill to medium heat. It's important to get the grill nice and hot so that the tilapia and veggies cook evenly. If using the oven, preheat it to 400 degrees Fahrenheit.
In a large bowl, combine the sliced summer squash, halved grape tomatoes, thinly sliced red onion, green beans, lemon juice, oregano, olive oil, ¼ teaspoon salt, and ¼ teaspoon pepper. Mix everything together gently until the vegetables are well-coated.
To make a packet, lay two 20x12-inch sheets of foil on top of each other. This double layering will protect the contents from burning. Spray the top of the foil with nonstick spray to prevent sticking.
Place one portion of tilapia in the center of the foil. Sprinkle it with a pinch of salt and pepper; then top with about ¾ cup of the vegetable mixture. If desired, sprinkle a few capers and olives on top.
Bring the short ends of the foil together and seal tightly, leaving enough room for steam to gather. Pinch the seams along the sides to ensure nothing leaks out.
Repeat the process with the remaining foil, fish, and vegetable mixture until you have made four packets.
Place the packets on the grill and cook for 5 to 10 minutes. Time may vary depending on the thickness of the tilapia. You’ll know they’re done when the fish is opaque and flakes easily with a fork, and the vegetables are tender.
Once cooked, carefully open both ends of the packets to allow the steam to escape. Use caution as the steam will be hot!
Use a spatula to slide the contents onto plates, and serve immediately while hot.
