Place a large pan over a medium heat and add 2 tbsp of vegetable oil.
Peel and chop your potatoes and boil in a large saucepan for 25-30 minutes until completely soft. Drain and return the cooked potatoes to the large saucepan and set aside.
Add your diced carrot/swede and fry for 4-5 minutes until slightly softened.
Add your beef mince and fry until nicely browned.
Add in the flour and stir until it disappears.
Add the thyme, tomato puree and crumble in a stock cube. Stir once again.
Add in 500ml of boiling water and gravy browning, if using. Season with a generous pinch of salt and a small pinch of pepper.
Allow to simmer for 10-15 minutes or until the veg is nicely softened.
Preheat your oven to 180C/200C (fan)
Add 50g of your grated cheese, all the butter and 1 tbsp of dijon mustard to the cooked potatoes in the saucepan. Use a potato masher to mash the potatoes until perfectly smooth. Season with a pinch of salt and pepper.
Once your pie filling it done, pour it into your roasting dish. Mine is 33x24cm. Pat it down into an even layer.
Spoon your potato on top and smooth over. Using a fork, create a lined pattern on top. This helps crisp up the top!
Sprinkle your remaining cheese on top.
Place into the oven for 40 minutes or until the top is lovely, crisp and golden.
Once cooked, sprinkle more thyme on top and a little flaky Maldon sea salt.
Serve up with boiled veg, gravy and enjoy!