Pre-heat oven to 200°C/400°F/Gas Mark 6. Melt the butter in a heavy based 23cm/9 inch ovenproof frying pan and fry the parsnip, carrot, sweet potato and red onion for 5 minutes. Stir in the stock and bring to the boil, simmer for 10 minutes until the vegetables are just tender.
Stir in the courgettes and basil and simmer for 4-5 minutes until all the stock has been absorbed. Stir in the balsamic vinegar and bubble until thickened to a sticky glaze.
Roll out the pastry on a lightly floured surface to a 24cm/9.5 inch circle/square. Remove the frying pan from the heat and transfer the vegetables to a baking tray. Lay the pastry over the vegetables, tucking in the edges. Bake for 20-25 minutes until risen and brown.
Carefully turn out the tart onto a serving plate and serve immediately with fresh rocket leaves piled on top.