Preheat the oven to 200°C. Roll out the pastry on a lightly floured surface to 1cm thick and cut a round roughly 10 inches in diameter.
Line a 7 inch springform, fluted tart tin using the pastry, pressing in well then prick the base. Trim any overhanging pastry and chill that tart tin for 10 minutes. Meanwhile, combine all the vegetables and thyme leaves with the olive oil and seasoning in a large mixing bowl.
Toss well to mix thoroughly. Arrange in the tart case and bake for 20-25 minutes until the pastry is golden and vegetables are tender and lightly coloured. Remove and sprinkle the Gruyère on top, then return to the oven for 5 minutes.
Remove and allow to sit for a few minutes before turning out on a wire rack and garnishing with the sprigs of thyme. Serve warm.