Tip the couscous into a large bowl along with the pesto and crumble over the stock cube. Pour over 2 teacups (about 400ml/14fl oz) boiling water and give it a good stir to dissolve the stock. Cover with cling film and leave to stand for
10 mins until all the water has been absorbed.
To make the dressing, mix together the olive oil, pesto and lemon juice. When the couscous has swelled up, pour the dressing over and flake through with a fork, breaking up any clumps. Toss in the mozzarella, tomato and rocket, then serve.