Peel and finely chop the onion. Trim the dry ends off the celery and finely slice it. Warm a large pan over a medium heat. Add the chopped veg and 1 tbsp olive oil. Season with a little salt and pepper. Give it a good stir, then turn the heat right down and cover. Sweat the veg for 10 mins, stirring once or twice, till the veg are soft but not coloured.
While the veg sweat, slice the roots off the celeriac. Peel off the remaining skin and any tricky bits of root. Finely chop it. Peel and crush the garlic.
Stir the celeriac and garlic into the pan. Put the lid back on and sweat for another 15 mins.
Dissolve the stock cube in 850ml boiling water. Peel, core and roughly chop the apple. Add it to the pan. Pour in the stock. Turn the heat up and bring the soup to the boil. Turn it down a little and simmer for 5-10 mins till everything is soft.
While the soup simmers, warm 2 tbsp oil in a small frying pan. Add the sage leaves and sizzle for 30 secs-1 min till slightly browned. Lift out of the pan and leave to drain on some kitchen paper.
Ladle the soup into a blender and blitz till smooth, or use a hand-held blender in the pan. Add more water if it needs it. Taste and add more salt and pepper if necessary. Ladle into warm bowls and top with the crisp sage leaves to serve.