1.Preheat oven to moderate (180°C / 160°C fan-forced).
2.Peel sweet potatoes; place in bowl of cold water with the lemon juice to prevent browning. Cut potatoes into thick slices, return to the water.
3.Line a shallow oven tray with baking paper. Drain potato, pat dry with absorbent paper. Place potato and onion on tray. Drizzle vegetables with olive oil; drizzle sweet potato only with maple syrup. Sprinkle with sea salt, bake for about 40 minutes or until tender and browned.
This recipe can be made up to six hours ahead; reheat just before serving.Note