Heat your oven to 220°C/Fan 200°C/Gas 7. Trim the base off the cauliflower and cut the cauli into bite-size florets. Peel the red onions and chop into wedges. Scatter both the cauli and onions into a large roasting tin and toss with 1 tbsp oil and a good pinch of salt and pepper. Slide into the oven and roast for 25 mins, till tender and caramelised.
Meanwhile, tip the pearl barley into a small pan. Stir in the raisins and a pinch of salt. Pour over 150ml boiling water. Set the pan on the hob and return to the boil, then turn the heat down and simmer for 20-25 mins till the grains are tender.
While the barley and veg are cooking, make your dressing. Squeeze the juice from half the lemon into a bowl and whisk in 1 tbsp oil and a pinch of salt and pepper.
Tip the cooked barley and roasted veg into a bowl and spoon over the dressing. Fold in the rocket leaves and pile onto plates. Serve with the mozzarella torn and scattered over the top.