Heat your oven to 180°C/Fan 160°C/Gas 4. Peel, halve and finely slice the shallots. Pop them in a large bowl. Peel the parsnip and coarsely grate it. Add to the shallots.
Peel and coarsely grate the carrot. Peel the apple. Quarter it and slice out the core. Coarsely grate it and add to the bowl. Give everything a stir to mix it all together.
Finely chop the parsley leaves and stalks. Finely grate the zest from the lemon. Add them to the bowl. Juice half the lemon and whisk it with 1 tbsp cider vinegar, 1 tsp olive oil and a good pinch of salt and pepper. Taste and add a squeeze more lemon juice if you think it needs it. Add to the bowl and toss to mix. This is your slaw. Set aside to mellow while you roast the salmon.
Finely chop the chervil leaves. Mix most of them in a bowl with 1 tbsp wholegrain mustard and a pinch each of salt and pepper.
Line a baking tray with baking paper. Lay the salmon on the tray, skin-side up. Spoon the mustard over the salmon, smoothing it over the top to create a crust. Slide into the oven. Bake for 10-12 mins till the salmon is dark pink and flakes easily when pressed with a fork.
While the salmon roasts, tip the couscous into a heatproof bowl. Pour in 150ml boiling water. Cover with a plate or cling film and let it soak.
Use a spatula to lift the salmon fillets off the tray. Pop them onto a couple of warm plates and serve with the slaw and couscous, garnished with the last of the chopped chervil.