Stab the potatoes a few times all over, then coat in a drizzle of olive oil and a pinch of salt. Pop in the oven at 356F/180C for 80mins, or until deep golden, crispy and knife tender.
Once cool enough to handle, slice off the top (3/4 of the way up) and scoop out most of the centre into a large bowl, leaving a thin wall to keep the potato sturdy. Mash the potato with butter, milk, nutmeg, half of the cheese and plenty of salt & pepper to taste (I typically use 1/2 tsp salt & 1/4 tsp pepper).
Meanwhile, in a large pan over medium heat add a drizzle of oil alongside the carrot, onion, garlic and mushrooms. Fry until soft and starting to pick up colour, then add in the beef and fry until brown all the way through, breaking up with your wooden spoon as you go.
Stir in 1 tbsp tomato puree and fry for a couple of mins, then pour in the wine. Simmer for a few mins to allow the beef to soak it up, then pour in the beef stock. Stir in rosemary, thyme, gravy granules, Worcestershire sauce and salt & pepper (to taste). Reduce heat to low and gently simmer for around 20-25mins to thicken the sauce.
Spoon the beef into the potatoes, then top with a few heaped tbsp of potato. You should be left with 1 cup of mash, save it in the fridge for a rainy day. Top potatoes with cheese, then place back in the oven at 220C/430F for around 12-15mins, or until golden & bubbly!