Preheat your oven to 190 degrees.
Heat the olive oil in a large saucepan over medium/high heat. Add garlic, bacon and brown onion and cook for a couple of minutes until bacon starts to brown.
Add the beef mince, and using a wooden spoon, break up mince and combine with other ingredients. Cook for 5 minutes, stirring every so often to break up any clumps of mince.
Once mince has started to brown, add carrots, stock, mixed herbs, tomato paste and crushed tomatoes. Allow to simmer for 5 minutes before reducing to a low heat and allow to cook for a further 30 minutes.
Meanwhile, peel and cut potatoes into halves and cook in a steamer for 20 minutes (or until cooked through).
While potatoes are still hot, mash (I use a potato ricer) until smooth, add milk, parmesan cheese, butter and salt and pepper to taste.
Remove mince mixture from heat, and transfer to a casserole dish. Add the frozen peas and sliced mushrooms and mix together.
Spoon mashed potato over the meat mixture and sprinkle grated cheese over the top.
Place into oven and bake for 40 minutes, or until the cheese has melted and is beginning to brown.
** I have frozen this before, and it has reheated well. I'd suggest freezing for a maximum of one month.