In a kettle, combine the sweet potatoes with enough water to cover by 1 inch and boil for 35 minutes, until tender.
Drain the sweet potatoes and let them cool until they can be handled. The sweet potatoes may be prepared in advance and kept chilled overnight.
Peel the sweet potatoes, cut them lengthwise into sixths, and cut crosswise into 1/2-inch pieces.
Peel, core and cut the apples lengthwise into sixths and crosswise into 11/2-inch pieces.
In a large heavy skillet cook the apples in the butter over moderate heat, stirring, for 3 minutes. Stir in the lemon juice, the brown sugar, the cinnamon and the sherry. Bring to a boil, and simmer 3 minutes. Add the sweet potatoes and cook, stirring gently, for 2 minutes, until heated through.