Heat the oil in a frying pan over a medium heat and cook the onions for 4-5 minutes, or until soft and golden browned. Remove and set aside.
Heat the remaining oil in the same frying pan and add the minced lamb. Cook the lamb over a medium heat, stirring occasionally, for 7 minutes.
Stir in the flour and cook for a further 1 minute. Break up any lumps with a fork. Remove from the heat and add the beef stock, stirring until blended. Bring back to the boil and stir occasionally until thickened.
Add the cooked onions, tomato paste, worcestershire sauce, bay leaves, and thyme or dried rosemary. Add salt and pepper to taste and set aside to cool.
Meanwhile, mash the potatoes with the butter and a little milk until smooth. Season well with salt and pepper.
Preheat the oven to 350 ° F (180 ° C).
Transfer the meat mixture to a large ovenproof dish. Spoon over the mashed potato and fluff with a fork. Top with some extra butter.
Bake for 30 minutes or until golden brown and bubbling.