Preheat the oven to 200˚C fan/ 220˚C/ 392˚F convection/ 428˚F
In a medium saucepan over a low heat, add the olive oil then cook the onion, carrots and celery until soft for about 10 minutes
Add the crushed garlic and finely chopped rosemary and cook for 1 minute before adding the tomato puree.
Add the chopped lamb, stock, Worcestershire sauce and season with salt and pepper, bring to a simmer and cook for 15 minutes.
Meanwhile, cook the potatoes:
Boil the potatoes for 15-20 minutes, until soft.
Drain well then add the butter, milk and salt and mash until smooth.
Place the lamb into the bottom of a 20 x 27 cm (7.8 x 10.5") or similar size, baking dish.
Spoon over the mashed potato and smooth.
With a fork, drag lines across the top - these will get extra crispy.
Place in the oven and bake for 20-25 minutes until golden.
Eat straight away.