Melt the butter and 1 tbsp of the olive oil in a large pan and add the celeriac and 2 of the sprigs of rosemary with a large pinch of salt. Cook gently for 15 minutes – you don’t want to get any colour on the celeriac. If it starts to catch, add a splash of water. Add the garlic and cook for another minute. Pour in the chicken stock and simmer for 10 minutes or until the celeriac is very soft. Add the milk and scoop out the rosemary stalks and discard. Use a hand blender to whizz until smooth. Season and keep warm.