Add 2 tsp cumin seeds to a dry pan. Toast over a medium heat for 2-3 mins, shaking the pan, till they smell aromatic. Tip into a mortar and pestle. Set a pinch aside. Grind the rest to a powder.
Trim the leek. Slice a cross into the top and rinse out the grit. Finely slice it. Peel and finely chop the Jerusalem artichokes. Warm a large pan for 1 min over a medium heat. Add 1 tbsp oil and the veg. Season. Cover. Sweat over a low heat for 15 mins till soft and the Jerusalem artichokes are browned.
While the veg sweat, crumble the stock cube into a heatproof jug. Stir in 900ml boiling water to dissolve it (or use hot homemade stock).
Chop most of the sprouts, keeping 3-4 back for later. Peel, core and chop the apples. Stir the sprouts, apples and ground cumin into the veg. Add most of the thyme sprigs. Pour in the hot stock. Bring the soup to a boil. Simmer for 5 mins till the veg are soft.
Meanwhile, finely slice the remaining sprouts. Warm a frying pan. Add 1 tbsp oil and the sprouts. Fry over a high heat for 2-3 mins till crisp and a little charred. Drain on kitchen paper.
Lift the thyme sprigs out, ladle the soup into a blender and blitz till smooth (or use a hand-held blender in the pan). Taste and adjust the seasoning. Serve in warm bowls topped with the frizzled sprouts, cumin seeds and a few thyme leaves.
Lift the thyme sprigs out of the soup. Ladle the soup into a blender and blitz till smooth (or use a hand-held blender in the pan). Taste and adjust the seasoning. Serve the soup in warm bowls topped with the frizzled sprouts, toasted cumin seeds and a few thyme leaves.