Boil the peeled and chopped potatoes in salted water for 15 to 20 minutes until tender.
Brown the lamb in the oil over a high heat for 5 to 8 minutes and spoon off excess fat.
Add the onion, carrots, celery and garlic and cook for 8 minutes until soft.
Stir in the tomato purée and flour, then add the stock, Worcestershire sauce and thyme. Simmer for 25 minutes, adding the peas for the last 5.
Drain the potatoes, steam dry and mash with the butter and milk.
Preheat the oven to 200°C (180°C fan, gas 6). Top the filling with mash, rough it up with a fork, sprinkle with cheese and bake for 25 to 30 minutes until golden.