Place a large non-stick saucepan over medium heat. Spray with cooking spray and add the mince, onions, celery and carrots. Cook for 10 minutes stirring regularly so as not to burn. Break up the mince with the wooden spoon.

Stir the tomato puree, beef stock, Worcestershire sauce and mixed herbs. Season with salt and pepper. Bring to the boil then reduce heat and cover with lid, simmer for 50-60 minutes, stirring occasionally until meat is tender.

About 30 minutes before the meat is ready make the potato topping. Boil the potatoes in salted water until soft and tender. While the potatoes are boiling, heat the oil in a non-stick frying pan and fry the leeks for 8-10 minutes stirring regularly. Drain the potatoes well and return to the pan. Season with salt and pepper, add the creme fraiche until smooth then stir in the leeks and set aside.

Preheat oven to 200c. In the last 5 minutes of the meat cooking, mix the cornflour with the water to make a paste then add to the meat mixture. Cook for another 5 minutes or so until sauce has thickened.

Pour the meat into a shallow casserole dish then top with the mashed potatoes. Gently level the mash over the meat with a spoon, then fluff using a fork.

Bake for 30 minutes or until the potato topping is golden brown.

Let it rest for 15 minutes before serving. Serve with baked beans or side salad.