Preheat oven to 230°C. In very large non-stick frying pan, heat butter over moderate heat. Add onion and sauté for 5 minutes or until tender.
Add sweet potatoes, apples, 2 tablespoons sugar and lemon juice, and cook until sugar has melted. Add stock and pepper, and bring to boil.
Transfer mixture to a 19 × 28 cm glass baking dish. Cover with foil and bake for 25 minutes or until sweet potatoes are tender. Uncover, sprinkle with remaining 1 tablespoon sugar and bake for 10 minutes or until lightly browned.