Heat the oil in a large saucepan over a medium heat, tip in the celery, garlic and potatoes and coat in the oil. Add a splash of water and a big pinch of salt and cook, stirring regularly for 15 mins, adding a little more water if the veg begins to stick.
Pour in the vegetable stock and bring to the boil, then turn the heat down and simmer for 20 mins further, until the potatoes are falling apart and the celery is soft. Use a stick blender to purée the soup, then pour in the milk and blitz again. Season to taste. Serve with crusty bread.