Heat the oil in a flameproof casserole, add the lamb a few pieces at a time, then add the onion. Fry for about 5 minutes, stirring until browned all over.
Add the garlic and walnuts and fry for a couple of minutes more until the nuts are lightly toasted. Stir in the flour then add the stock, wine, tomato purée, bouquet garni and a little seasoning. Bring to the boil, stirring occasionally. Cover and cook in a preheated oven, 160°C (325°F), Gas Mark 3, for 1½ hours or until the lamb is tender. Allow to cool then chill until required.
When ready to serve, cook the pasta in a large saucepan of boiling water for 8–10 minutes until just tender. Reheat the lamb on the hob, stirring until piping hot. Drain the pasta then toss with the lamb and sprinkle with chopped parsley and a few extra walnuts. Spoon into shallow bowls. Serve with a salad of rocket, watercress and spinach dressed with lemon juice and sprinkled with Parmesan shavings.