Heat a medium (26 cm/10-inch large) oven-friendly skillet with 2 tablespoons of olive oil over low heat. Fold 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 beef and cook, stirring often, until it's browned, then scoop out with a spoon any excess fat.
Add back in the veggies, then add the Worcestershire sauce, followed by the tomato paste and thyme 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, 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 to rest for 5 minutes before serving.