Peel and finely chop the onion and carrot. Slice the celery. Warm a large pan for 1 min. Add ½ tbsp oil and the chopped veg. Season. Sizzle over a medium heat for 5 mins till the veg pick up a little colour.
Crumble a stock cube into the pan. Pour in 500ml boiling water (or use homemade stock). Cover and simmer for 10 mins.
Thinly slice the mushrooms. Peel and finely chop the garlic. Warm another pan for 1 min over a medium heat. Add ½ tbsp oil and the mushrooms. Season. Fry for 8-10 mins till golden. Stir now and then. Finely chop 1 tbsp rosemary. Add to the mushrooms with 1 tbsp balsamic and the garlic. Cook over medium-high heat for 1-2 mins till fragrant.
Take off the heat. Scoop out a heaped spoonful of mushrooms and set aside.
Peel, core and chop the apple. Add the mushrooms and apple to the soup pan. Blend till smooth with a hand-held blender, or blitz in a blender. Taste and season. Ladle into warm bowls. Garnish with the reserved mushrooms and extra rosemary.