Shepherd’s Pie

Name: Shepherd’s Pie
Yield: Serves 6
Ingredients: 375 ml (13 fl oz/1½ cups) chicken stock; ½ teaspoon dry mustard; 750 g (1 lb 10 oz) lean cooked roast lamb, trimmed of fat and finely chopped; 4 large potatoes; 2 tablespoons worcestershire sauce; 2 tablespoons seasoned flour; 30 g (1 oz) butter, extra; 125 ml (4 fl oz/½ cup) hot milk; 25 g (1 oz) butter; 2 onions, finely chopped