Pour the couscous into a large heatproof bowl. Dissolve the stock cube in the measured boiling water, then pour the boiling stock over the grains, cover and set aside for 10–15 minutes to swell up.
When all the liquid has been absorbed, use a fork to fluff up the grains, then douse generously with one-third of the oil. Now add the almonds, caper berries and fennel and toss everything together well.
Add the tuna, breaking it up with a fork and mixing it through the salad. This will happily keep for a day in the fridge.
Lastly throw in the rocket (if it sits in the dressing, it will wilt). Squeeze in the juice of the lemon and dress with what is left of the oil. Toss again and add salt and pepper to taste.