Preheat the oven to 350ºF. Peel and quarter the onion and separate into petals, then scatter into a 36 x 25 centimeter roasting pan. Seed and roughly chop the pepper, halve the tomatoes and roughly slice the zucchini, then add to the pan. Pick over the oregano leaves, drizzle with two tablespoons of oil, season with sea salt and black pepper, then roast for 50 minutes, tossing halfway.
Remove the pan from the oven and push the veg in towards the center slightly. Quickly and carefully rub the sides and corners of the pan with oil, then confidently cover the veg with the pastry sheet and tuck in the edges. Place back in the oven for 30 minutes, or until golden and puffed up. Gently run a palette knife around the edge of the pastry to loosen, then carefully turn out onto a board. Tear over the mozzarella and basil, and drizzle with the thick balsamic, to taste.
You can easily ramp up your veg by serving with a spinach salad.