Heat oven to 400°. In a bowl, toss apples with walnuts, ginger, 2 tbsp sugar, flour and cinnamon.
Roll piecrust into a 12-inch circle, about 1/8-inch thick. Coat a baking sheet with cooking spray; place piecrust on sheet. (It's OK if the piecrust hangs over the sides.)
Arrange apple slices in one layer to form a circle about 3 inches from edge of piecrust; fill in circle with more slices.
Scrape any remaining walnuts and ginger from bowl onto apples. Gently fold edges of piecrust over outer edge of apple filling to form a thick crust.
Spray outer rim of crust with cooking spray; sprinkle crust with remaining 2 tsp sugar.
Bake until crust is golden brown and apples are soft, 35 minutes. Cut into 8 slices and serve.