Preheat the oven to 180°C.
Slice the drained beetroot into wedges and add to a roasting tin. Peel the onions, and cut into quarters through the root. Leaving the root intact, cut each quarter in half again, giving you eight wedges from each onion. Add to the roasting tin, along with the halved chestnuts. Drizzle over the olive oil and tuck in the rosemary.
Season generously with salt and pepper and roast in the oven for 25 minutes, stirring to turn over the vegetables about half way through cooking.
Remove from the oven and pour over the maple syrup, tossing to coat, then return to roast for a further 10-15 minutes until the onion is soft when pierced with the tip of a sharp knife.
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