Begin by popping your carrots in boiling water for around 10 minutes or until soft enough to slice through. Meanwhile in a suitably sized pot/pan, saute your onions, garlic and mushrooms in olive oil over medium heat until they begin to soften. Add your mince, break it up and continue to fry over medium heat until brown all the way through. At this point your carrots will be done. Rinse them with cold water, finely dice and throw in the pot with the mince.
Now it's wine time. Pour in your red wine and leave to simmer for around 5 minutes to burn off the alcohol and allow the mince to soak up the flavour. After, add your beef stock, Worcestershire sauce, tomato puree, thyme, bay leaf, salt and pepper (to taste) and leave to simmer over low heat for a good 45 minutes, stirring occasionally. At this point it will look like you've got way too much liquid but it will naturally thicken, so fear not.
Whilst your mince is simmering, get started on the mash. Whack your peeled potatoes in a pot of salted boiling water for around 15 minutes or until soft enough to comfortably slice. Drain (don't rinse) and place back in the pot. Mash with your butter, nutmeg and generous helpings of salt and pepper until a smooth but fluffy texture. Leave to one side and pre-heat oven to 220 C / Gas 6.
In a tin or baking dish, firstly pour in your mince (removing bay leaf), then top the mince with a thin layer of cheese and then finish with your mash. Pop in the oven for around 20 minutes or until golden and crispy on top. Leave to sit for a few minutes to retain shape and then serve.