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In a very large saucepan, steam the sweet potatoes just until tender, about 12 minutes. Spread the steamed sweet potatoes in a large shallow baking dish and let cool slightly.
In a large skillet, melt the butter in the oil. Add the onions and cook over low heat, stirring occasionally, until softened, about 8 minutes. Add the curry powder and cook, stirring, until fragrant, about 4 minutes longer. Add the carrot juice and simmer until slightly thickened, about 3 minutes. Add the lemon juice and season with salt and pepper. Pour the curried onions over the sweet potatoes.
Preheat the oven to 425°. Bake the sweet potatoes in the upper third of the oven until hot, about 25 minutes, then serve.
The sweet potatoes can be prepared through Step 2 and refrigerated overnight. Bring to room temperature before baking.
Fresh carrot juice is available in health food markets and some large supermarkets.