Tip: place sticks of butter in the freezer for at least 15 minutes. Then use a grater grate the butter so that it spreads easily over the top of the cake.
Preheat oven to 350°F.
Pour pie filling, peaches, and blueberries in the bottom of a 9x13-inch pan. Top with powdered cake mix. Sprinkle brown sugar and nuts, if using, over the top.
Grate the frozen butter evenly over the top of the cake mix. Alternately, chop small pieces and spread them evenly. (Grating the butter gives easier and better coverage.)
Bake for 45-50 minutes. Serve warm or room temperature with ice cream. Store in refrigerator for up to 3 days.