Preheat your oven to 200°C/Fan 180°C/Gas 6. Trim the courgettes and cut them into bite-size chunks. Peel the red onion and slice it into thin wedges. Spread the courgettes and onion out on a large baking tray. Season with salt and pepper and add 1 tbsp olive oil. Toss to coat, then roast in the oven for 20-25 mins, till tender.
While the veg roast, fill and boil your kettle. Tip the couscous into a heatproof bowl and top up with 150ml boiling water from the kettle. Stir, cover the bowl with a plate and set aside for 10 mins to soak. The couscous will absorb the water and become tender.
Peel and grate the garlic into a bowl. Finely chop the chervil leaves and stalks and add them to the garlic. Grate in the lemon zest and squeeze in the juice from 1 half. Add 1 tsp mustard, 1 tbsp olive oil and some salt and pepper and whisk to make a dressing.
Halve the cherry tomatoes. Crumble the feta.
Divide the couscous between a couple of plates. Top with the roasted veg, the cherry tomatoes, crumbled feta and handfuls of rocket. Drizzle with the mustard and chervil dressing to serve.