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.