Preheat the oven to 180ºC, fan 160ºC, gas mark 4. Grease a large shallow ovenproof gratin dish with a small knob of butter.
Place the prepared vegetables and apple into a bowl. Season and toss together to mix.
Arrange the vegetable mixture in the dish and spread out evenly. Push sage leaves in between the ingredients.
Pour over the hot stock and top with the bacon pieces and dot with the remaining butter. Cover with foil and cook in the oven for 50 minutes or until the vegetables are tender.
Remove the foil and cook for a further 15 minutes to brown and crisp the top.
Cook's tip: Reduce cooking time in the oven by parboiling the vegetables for a few minutes before assembling the dish.