Have ready one 9-inch unbaked pie shell and a baking sheet. Place oven rack on lowest rack position in the oven. Heat oven to 450 degrees F.
TOPPING: Mix flour, brown sugar, cinnamon and walnuts in a medium bowl. Cut in butter until mixture forms moist, coarse crumbs that clump together easily.
FILLING: Peel, halve and core apples and cut in 1/8-in. thick slices. Place apples in a large bowl, add lemon juice and toss to coat. Mix remaining filling ingredients in a small bowl, sprinkle over apple slices and toss to coat.
Layer apple slices in pie shell, mounding them higher in center. Gently pat topping evenly over apples to form a top crust. Place pie plate on the baking sheet to cathch any drips.
Bake 15 minutes. Reduce oven temperature to 350 degrees F and bake 45 minutes longer, or until a skewer meets some resistance when center of pie is pierced (apples will continue cooking after pie is removed from the oven) and topping is golden brown. (If topping browns too quickly, drape a piece of foil loosely over the pie.).
Cool pie completely on a wire rack before serving.