Heat a large pan over high heat and add half of the oil. Brown the mince in batches so it browns evenly without letting out too much of its juices, thus boiling the meat. Drain the meat and set aside.
Heat the remaining oil over medium heat. Slowly soften the onion, carrots, and celery with the bay leaves, garlic, and rosemary.
Add the flour and tomato puree once the vegetables have softened and the onions are translucent. Cook for 1 minute more to cook out the flour, then add the beef mince back to the pan.
Add the ale, followed by stock and bring to a boil. Reduce to a simmer, cover, and cook until the gravy is thick and the vegetables are tender.
Preheat the oven to 200C or 180C for a fan-assisted oven or gas mark 6.
Meanwhile, bring a large pan of salted water to a boil, and cook the potatoes for 20-25 minutes until tender.
Drain through a colander and leave to stand for 2-3 minutes so the residual heat allows the potatoes to dry.
In a pan, melt the butter, warm the milk, and add the potatoes. If using a potato ricer, mash the potatoes before adding them to the pan. Mix until smooth. Add salt to taste
Once the beef is done, add the Worcestershire sauce, season to taste, and spoon into a casserole dish.
Cover with the mash and bake in the middle of the oven for 20-25 minutes until the topping is crisp and golden.
