Trim the tops and scoop out the center and the seeds of each apple and rub the edges of the trimmed fruit with the lemon. In a medium bowl, combine the butter, cereal, sugar, and walnuts. Fill the apples with the cereal mixture. Set the apples upright in muffin tins and bake until tender and bubbly, about 20 minutes. Transfer to serving dishes, top with ice cream and serve hot.