Preheat the oven to 180°C (350°F/Gas 4). Cut the shallot in half and then cut into slices lengthways, leaving the root intact. Arrange the slices on a non-stick baking sheet and sprinkle over the caster sugar. Bake for 5 minutes until lightly golden and caramelised. Remove from the heat and leave to cool completely.
Cover the potatoes with cold water in a pan and add a pinch of salt. Bring to the boil, then simmer for 15-20 minutes or until completely tender when pierced with the tip of a sharp knife. Drain in a colander and peel the potatoes while they are still hot.
Meanwhile, heat the olive oil in a pan. Add the onion, leek, carrot and garlic and sauté for 3-4 minutes until just beginning to soften but not colour. Stir the mince into the pan and cook for 2-3 minutes until browned, breaking up any lumps with the back of a wooden spoon. Stir in the tomato puree and cook gently for another 10 minutes until completely tender. Season to taste and keep warm or reheat as needed.
Push the cooked peeled potatoes through a potato ricer or sieve using a spatula. Quickly heat the milk in a pan. Beat the butter into the warm mashed potato and then add enough milk to make a smooth puree. Season to taste.
Spoon the mince mixture into 6 cm/2 1/2 in ring moulds set on warmed serving plates. Carefully remove the moulds. Using two dessert spoons, shape the potato puree into quenelles and arrange three on top of each serving (or use a piping bag). Top with the caramelised shallot slices and drizzle around the herb oil. Garnish with the thyme leaves.