Peel and carrots and parsnips and cut into even-sized chunks. Trim the leeks and finely chop.
Heat a saucepan over a medium to high heat. Add the oil and butter and swirl until the butter has melted. Tip in the leeks and sauté for 2-3 minutes until softened.
Add the carrots and parsnips, then season with salt and pepper and sauté for another 5 minutes. Add the thyme, honey and vinegar, stirring to combine, then pour in the stock. Cover with a piece of parchment and then a lid. Reduce the heat to low and continue to cook the vegetables for 20 minutes until tender.
Carefully remove the lid and parchment and using a hand blender blitz the soup until smooth. Taste and adjust the seasoning, if necessary. Ladle into bowls and serve with some porridge bread, if liked.