Preheat and prep: Heat the oven to 400°F (200°C) or preheat your grill to medium-high.
Tear four large sheets of heavy-duty foil, about 12x18 inches each.
Par-cook potatoes (if using): If adding potatoes, microwave them for 3–4 minutes until just starting to soften. This ensures everything cooks evenly.
Season the chicken: In a bowl, toss chicken with 1 tablespoon olive oil, 1 teaspoon Italian seasoning, half the garlic, 1/2 teaspoon salt, and a few grinds of pepper.
Season the veggies: In another bowl, toss zucchini, peppers, onion, tomatoes, and potatoes with remaining olive oil, Italian seasoning, garlic, salt, pepper, and red pepper flakes if using.
Build the packs: Lightly spray each foil sheet. Divide the veggies among the sheets, then top with chicken.
Drizzle each with a splash of balsamic if you like.
Seal tightly: Fold the long sides of the foil together and crimp to seal, then fold and crimp the ends. Leave a little space inside for steam circulation.
Cook: Bake on a sheet pan for 22–28 minutes, or grill for 18–22 minutes, flipping once halfway. Cook until the chicken hits 165°F in the thickest piece.
Finish and serve: Carefully open the packs—watch the steam.
Sprinkle with Parmesan and fresh herbs. Taste and adjust salt as needed.