Put the lamb in a bowl and add the garlic, onions, red pepper, herbs and wine. Mix well. Cover loosely with clingfilm and marinate in the refrigerator overnight.
Drain off the liquid thoroughly from the lamb and vegetables; reserve the liquid. Pat the lamb and vegetables dry on kitchen paper. Melt 25 g (1 oz) of the butter with the oil in a large frying pan on the hob and fry the lamb, in batches, until browned. Transfer to a casserole dish. Gently fry the vegetables for 5 minutes and add to the casserole.
Melt the remaining butter in the frying pan and add the flour, stirring to make a paste. Gradually whisk in the reserved liquid and bring to the boil. Pour into the casserole dish and cover with foil. Place on the lower rack of the halogen oven.
Set the temperature to 200°C (392°F) and cook for 30 minutes. Stir in the mushrooms and cook for a further 20 minutes or until the lamb is tender. Check the seasoning, adding salt and pepper if needed, and sprinkle with finely chopped parsley. Serve with mashed potato, saffron risotto or polenta.