Heat your oven to 200°C/Fan 180°C/Gas 6. Peel the onion and slice it into thick wedges. Halve the pepper. Scoop out the seeds and white bits, then thickly slice. Halve and thickly slice the courgette.
Chuck all the veg into a roasting tin. Peel and finely chop the garlic. Finely chop the parsley stalks (keep the leaves for later).
Add the garlic and parsley stalks to the veg with ½ tbsp oil, a pinch of 2 of harissa (it’s spicy, so use as much or as little as you prefer) and plenty of salt and pepper. Stir to coat the veg in the oil and seasoning. Roast for 40 mins till the veg are tender and a little charred.
When the veg have roasted for 25 mins, tip the couscous into a heatproof bowl. Pour over 200ml boiling water. Cover the bowl with a plate and set aside. The couscous will absorb the water and become tender.
While the couscous soaks, rub ½ tsp oil and some seasoning into the steaks. Heat a griddle or frying pan till it’s smoking hot. Add the steaks. Fry for 3-5 mins on each side, depending on how well done you like your steaks.
Slide the steaks onto a warm plate. Loosely cover with foil. Leave to rest for 5 mins.
Pick any thick stalks out of the watercress so you just have tender leaves and sprigs. Stir the watercress and couscous into the roast veg. Finely grate the zest from the lemon and stir that in too. Add a squeeze of lemon juice. Serve the couscous with the steaks, garnished with the parsley leaves.