Preheat oven to 425. Combine apples, lemon juice and vanilla extract in large bowl; set aside.
Combine brown sugar, 3 tablespoons flour, cinnamon and nutmeg in medium bowl. Add brown sugar mixture to apple mixture; toss to coat. Spoon into pie shell; set aside.
Heat a 10" nonstick skillet over medium-high heat until very hot. Add walnuts and cook, stirring constantly, until lightly browned and fragrant, for about 2 minutes. Remove from heat; set aside.
Combine remaining 1/3 cup flour and granulated sugar in small bowl. Cut in butter using pastry blender until mixture resembles coarse crumbs; sprinkle evenly over pie. Top with walnuts.
Bake until pie is bubbly and apples are tender in center, about 35 to 40 minutes
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