Heat a large, heavy-based pan. Add the oil and then add the onions, celery, carrots, thyme, bay leaves and cook over a medium heat for about 10 minutes until the vegetables have softened and taken on a little colour, stirring occasionally.
Add the minced beef to the pan and cook until browned, breaking up any lumps with the back of a wooden spoon. Stir in the flour with the tomato puree, mustard and Worcestershire sauce, then gradually add the stock and bring to the boil. Reduce the heat and simmer gently for about 1 hour until the meat is meltingly tender, stirring every now and then to stop the bottom catching. Transfer to a large baking dish and leave to cool.
To make the cottage pie, preheat the oven to 190°C (375°F), Gas mark 5. Place the potatoes in a pan of boiling salted water and cook for 15-20 minutes or until tender. Drain well and then return to the heat for a couple of minutes to dry out completely. Mash well and then beat in the butter, milk and cheese. Cover the minced beef mixture with an even layer of the cheesy mashed potatoes. Bake for 25–30 minutes until bubbling and golden brown. Place directly on the table and allow everyone to help themselves.