Finely chop the chestnut mushrooms. Grate the mature Cheddar cheese.
- 1 Heat the oil in a deep frying pan over a medium heat. Add the beef mince, frying for 10 minutes, until caramelised. Add the mushrooms and continue to cook for a further 5 minutes. Season with a pinch of salt and pepper.
- 2 Add the tomatoes to the pan and simmer until almost all of the liquid has cooked out.
- 3 Add the beef gravy to the pan, bring to a simmer and season again with salt and pepper. Simmer gently for a further 15 minutes before pouring into a deep baking dish.
- 4 Allow to cool, then place the dish in the fridge. This will firm up the sauce and make it easier for you to spoon on the mash later.
- 5 Preheat the oven to 180°C/160°C fan/gas mark 4. Warm the mash in the microwave for 5 minutes, stir the cheese through it, and then spoon it over the mince in the baking dish.
- 6 Use a fork to create high tracks in the mash that will become crisp in the oven. Bake for 30 minutes, until golden on top.