Heat a medium (26 cm/10-inch large) oven-friendly skillet with 2 tablespoons of olive oil over low heat. Add in the carrot, celery and onion and cook, stirring often for about 5 minutes, until the veggies have softened. Then tranfer the vegetables into a bowl until needed.
Return the pot to medium heat, add in the lamb mince and cook, stirring often, until it's browned, pour in the wine and let it evaporate. Then scoop out with a spoon any excess fat.
Add back in the veggies, then add the Worcestershire sauce, followed by the tomato paste, thyme and rosemary sprigs, and cook, stirring all the ingredients together, for a further 2 minutes.
Pour the stock into the pot, bring to a gentle boil and simmer, covering the pot with a lid, for 20 minutes. Remove the lid and continue to cook for a further 10 minutes. Discard the thyme sprigs and season with sea salt and freshly-cracked black pepper to taste.
Meanwhile, preheat the oven to 180 C/ fan 160 C/ gas 4.
Remove the skillet from the heat, and top the meat with prepared mashed potatoes (see method below), then raffle the top with a fork. Alternatively, transfer the mashed potatoes into a piping bag and pipe on top of the meat.
Transfer the skillet into the oven and bake for 10-15 minutes, then place under the grill for a further 5 minutes. Remove from the oven and allow the Shepherd's Pie to rest for 5 minutes before serving.