Heat your oven to 200°C/Fan 180°C/Gas 6. Trim the beetroot and peel it. Coarsely grate it. Tip it into a large bowl. Set aside.
Line a baking tray with foil. Rub 1 tsp oil into each salmon fillet and season them with a little salt and pepper. Lay them on the foil, skin-side up.
Slide the salmon into the oven. Roast for 12-15 mins till opaque. The salmon will flake easily when pressed with a fork.
While the salmon roasts, tip the couscous into a heatproof bowl. Cover with 125ml boiling water. Place a plate over the bowl and set aside to soak.
Peel and crush or grate the garlic. Add it to the beetroot. Finely chop the dill, discarding any really thick stalks. Add to the beetroot, too.
Pour the yogurt into the bowl. Add 1 tbsp cider vinegar. Stir the yogurt, vinegar and beetroot together. Taste and add a little salt and pepper if you think it needs it. This is your beetroot tzatziki.
Fluff the couscous with a fork and divide between 2 warm plates. Toss the rocket with 2 tsp oil and some salt and pepper and heap up on the plates. Lay a salmon fillet on each plate and serve with the beetroot tzatziki.