Trim the very ends off the leek. Halve it, rinse out any grit and finely slice. Peel and finely chop the parsnip. Warm a large pan for 1 min over a medium heat. Add 1 tbsp oil and the veg. Season. Cover. Sweat over a very low heat for 15-20 mins till soft.
Add 2 tsp cumin seeds to a dry frying pan. Toast over a medium heat for 2-3 mins, shaking the pan, till they smell aromatic. Tip into a bowl. Crumble the stock cube into a heatproof jug. Stir in 900ml boiling water to dissolve it (or use hot homemade stock).
Pull any sad looking leaves off the sprouts. Chop most of them. Peel, core and chop the apples. Stir the chopped sprouts, apples and half the cumin into the veg. Add most of the thyme sprigs. Pour in the hot stock.
Bring the soup to the boil (uncovered). Simmer for 10 mins. 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 out the thyme sprigs. Ladle the soup into a blender and blitz till smooth (or use a hand-held blender). Taste and season. Serve topped with the frazzled sprouts, toasted cumin seeds and a few thyme leaves.