Preheat the oven to 200°C, 400°F, gas mark 6. For the pie, finely chop the onion, leek and carrot. Melt butter in a frying pan and gently fry the vegetable mixture for 8-10 minutes until tender. Remove from the pan and set aside.
Add the turkey mince to the pan, turn up the heat and fry for about 5 minutes. Add the vegetable mixture, the Worcestershire sauce, peas, tomatoes, tomato puree and thyme. Stir and simmer for 20 minutes until reduced.
Meanwhile, for the mash, cut the potatoes into even-sized chunks and cook in boiling, salted water for 20 minutes. Drain and mash with butter and seasoning. Stir in half the Cheddar.
Season the mince mixture and spoon into a shallow ovenproof dish. Top with the Cheddar mash. Sprinkle over the remaining cheese and cook for 30 minutes until golden brown and hot.
For the pie:
For the mash: