Peel and finely chop the onion. Peel and crush or finely grate the garlic. Trim and peel the carrot, then chop it into small pieces.
Pour 1 tbsp oil into a pan and bring to a mediumhigh heat. Pick the sage leaves off their stalk. When the pan is hot, scatter in the sage leaves. Fry for 1 min, till crisp, then scoop them out of the pan and transfer to a plate.
Turn the heat under the pan down to medium and add the onion, garlic and carrot. Sprinkle in a little salt and pepper. Fry for 4-5 mins, till softened and golden, adding a splash of water if the onion starts to stick.
Meanwhile, chop the cauliflower into small florets, and finely slice the stalk, discarding the leaves. Cut the apples into quarters and slice out the cores. Roughly chop the apples.
Add the cauliflower and apples to the pan. Crumble in the stock cube. Pour in 750ml boiling water. Stir to combine, then bring to the boil. Pop on a lid, reduce to a simmer and cook for 20 mins.
Use a stick blender to whizz the contents till smooth. Alternatively, carefully pour into a blender (in batches, if necessary) and blend.
Pour the soup into deep bowls, top with the crisp sage leaves and a good crack of black pepper.