Blend the breadcrumbs, parsley and garlic in a food processor until the mixture resembles fine breadcrumbs.
Add the olive oil and Dijon mustard and season, to taste, with salt and white pepper. Blend again until the mixture resembles a smooth paste. (You may need to add a little water.)
Place the mixture onto a large sheet of greaseproof paper, cover with another sheet of greaseproof paper and roll out until large enough to cover the leg of lamb. Set aside.
For the lamb, preheat the oven to 190C/375F/Gas 5.
With a sharp thin knife or a metal skewer, make several small incisions into the flesh of the lamb. Press the garlic slices into the holes.
Place the rosemary and thyme sprigs into the leg of lamb where the bone has been removed and season, to taste, with salt and freshly ground black pepper. Roll the lamb up into a cylinder shape and tie with string.
Heat the olive oil in an frying pan and fry the leg of lamb, turning frequently, for 4-5 minutes, or until browned all over.
Place the reserved bones into a flameproof roasting tin and top with the leg of lamb. Roast in the oven for 40-45 minutes (for medium), or until cooked to your liking.
Meanwhile, for the roasted root vegetables, place the vegetables onto a baking tray. Drizzle the olive oil over the vegetables and sprinkle over the dried oregano. Season, to taste, with salt and freshly ground black pepper.
Roast in the oven for 20-25 minutes, or until golden-brown and tender.
Remove the lamb from the oven and set aside to rest for 30 minutes. Remove the string.
Meanwhile, preheat the grill to high.
Transfer the leg of lamb to a clean roasting tray, brush with half of the mustard and cover with the sheet of herb crust. Grill for 8-10 minutes, or until the crust is golden-brown.
Meanwhile, place the roasting tray used to roast the lamb onto the heat and stir in the remaining Dijon mustard and chicken stock. Bring the gravy to the boil and remove from the heat. Season, to taste, with salt and freshly ground black pepper.
To serve, carve the lamb into slices. Spoon some roasted root vegetables onto each of 8 serving plates, top with a couple of slice of lamb and spoon over some gravy.
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