1 In a medium bowl, use a fork to mix together oats, walnuts, brown sugar, flour, cinnamon and ginger. Drizzle with butter and combine to form a coarse crumb mixture. Set aside.2 In a large bowl, toss apples with sugar, lemon juice, cinnamon and ginger.3 Arrange apples in prepared pie crust. Sprinkle walnut streusel over top, completely covering the apples.4 Bake pie on a baking sheet, covered loosely with foil in centre of a 375°F oven for 50 minutes.5 Remove foil and bake until streusel is golden and apples are tender, about 10 minutes more.6 Let sit 15 minutes before serving.