Preheat the oven to 200ºC/gas mark 6. Put the potatoes in one roasting tray and the courgettes and onion in another. Drizzle each with half the olive oil, season and toss to coat. Bake for 30 minutes.
Meanwhile, make 2-3 slits in the skin of each salmon fillet, then stuff with small pieces of the lemon peel and the thyme and parsley sprigs.
Remove the trays from the oven. Add the potatoes to the courgettes and onion, then pour over the chopped tomatoes and scatter over the olives and half the basil. Top with the salmon fillets, skin side up. Return to the oven and cook for 10-15 minutes until the salmon is cooked through. Remove from the oven and serve garnished with the remaining basil.