Shepherds pie

Name: Shepherd's pie
Ingredients: 50g/2oz dripping; 2 large onions, finely chopped; 2 tbsp olive oil; 900g/2lb minced lamb; 2 tbsp plain flour; 4 bay leaves; 4 sprigs fresh thyme; 2 tsp anchovy essence; 2 small tins chopped tomatoes; 4 tsp worcestershire sauce; 450ml/16fl oz chicken, beef or lamb stock; Salt and freshly ground black pepper; 700g/1½lb potatoes; 55ml/2fl oz milk; 75g/3oz butter; 1 free-range egg yolk