Finely chop the onion, carrots, celery and rosemary leaves.
Heat the oil in a heavy-based saucepan or casserole dish, and gently soften the chopped vegetables for a few minutes.
Add the lamb, turn up the heat, and stir until the mince gets brown. Tip off any excess fat, then add the tomato purée and Worcestershire sauce and stir for a minute or two.
Pour in the stock, add one bay leaf and bring to a simmer. Cook under cover for 20 minutes, then uncovered for 10-15 minutes until you get a thick sauce.
Preheat the oven to 180°C. Meanwhile, boil the potatoes in lightly salted water, with an added bay leaf, for 10-15 minutes until tender. Drain, remove the bay leaf, then mash the potatoes with the butter, adding enough milk to get your desired creaminess.
Spoon the mash on top of the mince, ruffle with a fork. Alternatively transfer the mince to ovenproof ramekins or a large shallow dish and top with the mash.
Scatter some cheese on the top, if you like, and bake the pie or pies for 20-25 mins until the top is light brown and the mince is bubbling around at the edges.