Place a large pot on medium heat and add in the lamb mince. There's no need to add in oil, as the mince provides enough fat. Fry for 10 minutes, leaving the mince to brown on the base of the pan, creating a flavoursome fond. Then scrape that occasionally using the back of a wooden spoon.
After 10 minutes, add in the onion, garlic, celery, and carrot. Stir fry for 5-10 minutes until the onions have softened. Season with salt and pepper.
Time for the herbs, add in your thyme, bay leaves, and rosemary. Followed by the tomato puree and 1 tablespoon flour. Give it a good mix, then pour in the red wine, beef stock, and Worcestershire sauce. Give everything a good stir, bring to the boil, then reduce to low heat and leave to simmer for 45 minutes. Stirring occasionally to stop the sauce from sticking to the base of the pan.
Meanwhile, make your mashed potato. Add your chopped potatoes into a pan and top with cold water. Bring to the boil, then reduce to simmer for 10 minutes until you can spike the potato with a fork and it goes through easily. Drain and add the potatoes back to the pan with the butter, milk, salt and pepper. I used a hand blender to create silky smooth mashed potato, but you can use a potato masher or a fork for fluffy potatoes. Once mashed, taste and season accordingly.
Preheat the oven to 160c fan / 355F. Once your lamb mince filling has reduced and thickened, give it a taste and season with more salt if need be. Remove the bay leaves and thyme sprigs, then evenly spread the lamb mince filling into the base of your baking dish.
Top with the mashed potato and spread it out into an even layer.
Sprinkle with cheddar cheese or parmesan, or a mixture of both! Bake in the oven for 45 minutes or until golden and bubbling.
Take your shepherd's pie out of the oven and leave to rest for 10 minutes. Then serve up between 6 plates and enjoy!