PREHEAT oven to 425°F (220°C).
REMOVE pie shells from package and place one shell on a baking sheet and set aside. Using a small paring knife, gently pry the second pie crust from the foil pan and place on a lightly floured work surface. Using a lightly floured rolling pin, gently roll the crust to about 11”(28 cm) in diameter. Cut the crust into ½ (10 mm) strips with a pizza cutter or a sharp knife.
SPRINKLE walnuts evenly over the bottom of the intact pie shell. Arrange apple slices over the walnuts.
COMBINE sour cream, ½ cup (125 mL) brown sugar, flour, cinnamon and nutmeg. Spread over the apples. Sprinkle with the remaining sugar.
WEAVE strips of pie crust on top of pie in lattice fashion. Trim ends of strips flush with edge of bottom crust. Tuck bottom crust over strips and flute edge with floured fork or fingers. Brush the egg wash over the top of the pastry. Bake in preheated oven for 15 minutes. Reduce heat to 350°F (180°C) and bake for 30-35 minutes or until apples are tender.