Put the oil, onions and celery in your largest saucepan and cook gently until softened. Add the carrots and potatoes for a few mins, then add all the remaining ingredients, apart from the milk, with 1 litre water. Bring to a simmer. Cover and simmer for 30 mins, then uncover and simmer for 20-30 mins.
Fish out the bay leaves and whizz the soup with a hand blender. Add the milk and as much water as you need. Season to taste, warm through and serve with the Cheesy hot cross buns (see 'goes well with'). Can be made and chilled up to 2 days ahead – or freeze for 1 month, then reheat gently.