Fry the rashers of bacon in the oil until crisp. Scoop out half the bacon and set aside until later. Add the onion and garlic to the pan, stirring until tender, then add the parsnips or mushrooms. Cook for another 5 mins, then crumble in the chicken stock cube. Add the milk, 600ml water, the thyme and chestnuts. Cover and simmer for 30 mins until the parsnip is tender. Blitz with a hand blender, then season to taste. Ladle into bowls and top with the reserved bacon.