1. Preheat the oven to 200˚C, gas mark 6. Simmer the parsnips in boiling water with a large pinch of salt for 10 minutes; drain. Unroll the puff pastry and use a rolling pin to roll it out widthways to about 28cm. Cut out a 28cm diameter circle; chill until needed.
2. Place a large (about 28cm diameter at the top and 25cm at the base), ovenproof frying pan over a medium heat and add the butter, maple syrup and vinegar. When the butter has melted, swirl the mixture around the pan to combine. Sprinkle with the salt and a little pepper and scatter the thyme leaves over the surface. Take the pan off the heat and arrange the chicory, cut-side down, in the base of the pan, then scatter over the blanched parsnips. Return to the heat and cook for 4-5 minutes.
3. Drape the pastry circle over the vegetables, tucking it in at the edges of the pan (take care as the pan will be hot). Bake for 25-30 minutes, or until the pastry is puffed and golden. Rest for 5 minutes, then top the pan with a large serving plate that will hold any juices from the tart. Using oven gloves, carefully flip the pan so the tart ends up on the plate. Squeeze over the lemon juice and scatter the extra lemon thyme over the top. Serve with a crisp salad, if liked.