Shepherd's Pie

Originally a thrift dish for using up the Sunday roast. Beef makes 'cottage pie'; lamb makes 'shepherd's.' · 1½ lb ground lamb (or leftover roast lamb, diced) · 1 large onion, chopped · 2 carrots, diced · 1 cup peas · 2 tablespoons flour · 1½ cups beef or lamb stock · 1 tablespoon Worcestershire sauce · Salt, pepper, sprig of thyme · 2 lb potatoes, peeled · ½ cup milk · 4 tablespoons butter