Preheat the oven to 180°C/Fan 160°C/Gas 4. Add the sausage halves, onion and apple wedges to a large baking tray and scatter over the sage leaves.
Whisk together the olive oil, honey and whole grain mustard in a small bowl and drizzle over the sausage, onion and apple mixture.
Roast in the oven for 35-40 minutes or until golden brown and sticky. Serve with creamy mashed potato.