Toss the meat with the flour to coat lightly. Heat 1½ tablespoons of the oil in a large flameproof casserole dish or a deep frying pan over a medium heat. Sauté the lamb in batches until browned all over. Remove using a slotted spoon.
Add the shallots to the fat left in the pan and sauté, stirring frequently, for 5–8 minutes until lightly browned. Add the garlic, carrots and celery to the pan and cook for an additional minute.
Stir in the wine, turn up the heat and cook until the wine has almost evaporated. Add the stock, Worcestershire sauce, mustard and bouquet garni. Return the lamb to the pan along with any juices. Bring to the boil, then reduce the heat, cover and simmer for 20 minutes.
Preheat the oven to 190°C. Stir the leeks into the stew, cover and simmer for 10 more minutes. Season to taste. Remove and discard the bouquet garni, then spoon the mixture into a 1.75 litre ovenproof dish and stir in the parsley.
Cut the filo pastry into squares between 9 and 12 cm, depending on the size of your filo sheets. Lightly brush each square on one side with a little of the remaining oil, then crumple them up loosely and place them, oiled-side up, over the filling. Bake for 25–30 minutes until the pastry is crisp and golden.