Stir fry onions, celery, carrot and leek pieces in the oil over medium heat for 4 or 5 minutes. Add the litre of chicken stock and lamb shanks. Add more chicken stock if shanks are not covered. Bring to the boil and simmer very very slowly for around 3 hours (or until cooked). Leave to cool in stock and refrigerate. This is best done the day before needed. Next day, remove solidified fat from top of the mixture then gently re-heat. Drain shanks and vegetables (keeping warm), and pour the liquid into a wide pan. Bring to the boil, add Dijon-style mustard and reduce by two-thirds. It should be a syrupy sauce. Remove from heat and whisk in cream and egg yolk. (Do not re-boil once egg is added, it will curdle). Serve shanks on creamy mash potato with vegetables of your choice, and cover in mustard sauce. Sprinkle with chopped parsley. Per serving: 255 Calories (kcal); 18g Total Fat; (59% calories from fat); 5g Protein; 22g Carbohydrate; 108mg Cholesterol; 134mg Sodium Food Exchanges: 0 Grain(Starch); 0 Lean Meat; 3 1/2 Vegetable; 0 Fruit; 3 1/2 Fat; 0 Other Carbohydrates Converted by MM_Buster v2.0n.