In large bowl, place peaches and nectarines. Sprinkle with sugar, cornstarch and lemon juice; mix lightly with spoon until fruit is coated. Spoon into baking dish.
In large bowl, stir cookie mix, butter, vanilla and egg with spoon until soft dough forms. Spoon dough by heaping tablespoonfuls over fruit mixture, covering most of the top.
Bake 35 to 40 minutes or until filling is bubbly and topping is golden brown. Cool 10 to 15 minutes. Serve warm.