Sour Cream Apple Pie

1 refrigerated pie crust for two crust pie
2/3 cup sour cream
1/3 cup sugar
1 egg, lightly beaten
1/4 tsp. salt
1 tsp. vanilla
3 Tbsp. flour
5 or 6 tart apples, peeled and sliced

Topping: 

3 Tbsp. brown sugar
3 Tbsp. sugar
1 tsp. ground cinnamon
1 cup coarsely chopped walnuts

Line a greased 9-inch pie pan with crust. Preheat oven to 350 degrees. Whisk all filling ingredients, except apples, together in a bowl. Add apples, then spoon into the pastry-lined pie pan. Combine topping ingredients and sprinkle over filling. Cut remaining crust into 1/2-inch wide strips and arrange lattice fashion over apples. Trim edges of strips, tuck crust under and crimp edges decoratively. Bake pie in center of oven until the juices are bubbling and the apples are tender, about 55-60 minutes. Serve warm or cooled.