Heat oven to 400 degrees F. Line a large baking sheet with nonstick foil. Set aside.

Melt butter in a 10-inch nonstick skillet over medium to medium-high heat. Add apple slices and saute for 5 minutes to brown slightly, stirring occasionally.

Stir in sugar, cherries and cinnamon. Reduce heat to medium-low and cook about 2 minutes until sugar is dissolved. Stir in cornstarch mixture and cook 1 minute until slightly thickened. Remove from heat and stir in walnuts.

Unfold pastry sheet. Cut in half lengthwise, parallel to the fold. Working quickly on a lightly floured surface, roll out one pastry half to a 12 x 5-inch rectangle. Transfer to prepared baking sheet. Spoon apple mixture over pastry, leaving a 1-inch border around all edges.

Brush edges with egg white. Roll out second half of the pastry to a 12-1/2 x 6-inch rectangle. Fold pastry in half along the length and then cut slits in dough, but not all the way through to edge. Transfer on top of filling, then unfold. Pinch together at edge.

Brush pastry with egg white and sprinkle with granulated sugar. Bake at 400 degrees F for 18 to 20 minutes. Cool slightly before slicing and serving.