Fill and boil your kettle. Tip the pearl barley into a small pan. Pour in enough hot water from the kettle to cover the barley by a few cms. Add a pinch of salt. Bring to the boil, then reduce to a gentle simmer. Cover with a lid and cook for 20-25 mins, till the barley is tender.
Warm a small frying pan on a medium-high heat. Tip in the cashews and toast them for 3-4 mins, tossing frequently, till golden. Tip the cashews into a sturdy bowl. Use the end of a rolling pin or a jam jar to crush them slightly (you can use a mortar and pestle if you have one handy). Set aside.
Trim any thick stalks off the sprout tops and roughly shred the leaves. Slide the leaves into a large bowl. Peel and thinly slice the red onion. Quarter the apples and slice out the cores. Chop the apples into thick matchsticks. Add the red onion and apples to the sprout tops.
Peel and grate the garlic into a small bowl. Grate in the lemon zest and squeeze in the juice. Add 1 tsp Dijon mustard and 1 tbsp olive oil. Season with a salt and pepper and stir to make a dressing. Pour the dressing into the bowl of veg, tip in the cashews and fold together.
Drain the barley, if needed, and add it to the salad. Fold through the peppery salad leaves and serve.