First, make the puff pastry. Sift the flour into a bowl with the salt. Stir in the water, vinegar and melted butter. Mix until it forms a soft ball and then wrap in clingfilm and refrigerate for 30 minutes.
Remove the pastry from the clingfilm, flour the counter and roll the dough into a large square shape Place the slab of butter in the centre and fold the pastry over the butter completely, wrap in clingfilm and leave to chill for 20 minutes.
Roll the pastry into a narrow rectangle shape about half an inch thick. Fold one corner over to about one third of the length and then fold the remaining pastry over that, wrap and refrigerate again for 20 minutes. Repeat this process at least 6 times. The more the pastry is folded and the butter spread throughout the dough the lighter it will be. After final rolling leave in the fridge for at least 30 minutes before wrapping in the lamb.
For the Roasted Vegetables:
On a roasting tray, place the chopped parsnips, carrots and potatoes, the olive oil, garlic and rosemary. Drizzle over the honey and soy sauce. Roast in the oven 15-20 minutes until golden.
For the Duxelle:
In a pan, heat a little olive oil over a medium heat and add the onions, garlic until soft. Add the mushrooms and herbs and cook until all the liquid from the mushrooms has evaporated and set aside.
For the Lamb:
Season the lamb with salt and pepper and sear for one minute on both sides and set aside.
Roll out the puff pastry on a floured surface and cut into four strips. Spoon some of the mushrooms and spread over the pastry leaving an inch border.
Place the lamb on top and roll the pastry around it. Brush the pastry with a lightly beaten egg and place on a floured oven tray.
Bake in the oven for 15 minutes for rare lamb.
To serve, cut the lamb in half and place on a plate. Place the roasted vegetables on one side and serve with gravy.