Dust lamb with flour and season well. Heat half the oil in a large heavy-based casserole over medium heat. Cook lamb, in batches, for 5 minutes or until well browned. Transfer to a plate.
Add 1/4 cup of the stock to pan. Using a wooden spoon, scrape browned bits from base. Add onion, carrot and celery and cook, stirring occasionally, for 5 minutes or until stock has evaporated and onion has softened. Stir in mustard, sauce, ale and remaining stock. Bring to the boil.
Return lamb to pan and reduce heat to simmer. Cover and cook for 1–1 1/4 hours or until lamb is tender. Stir in parsley. Top with herbs.