Heat oven to 375 degrees F.
Melt butter in large heavy saucepan. Stir in apples. Stir in 1/2 cup granulated sugar; cook, stirring frequently, 12 to 15 minutes or until barely tender. Stir in 1/2 teaspoon vanilla. Remove from heat.
Combine walnuts, brown sugar, 1/2 cup flour, cinnamon and half the salt. With fingertips, rub butter into flour mixture until crumbly.
Mix sour cream, remaining granulated sugar, salt, flour and vanilla. Mix in nutmeg and egg.
Drain apples in colander; fold into sour cream mixture. Spoon into pie shell; tilt slightly so liquid flows evenly to all parts of pie shell. Top pie with streusel topping. Place on sheet pan.
Bake in 375 degree F oven for 50 to 60 minutes or until the apples are fork-tender. If the streusel topping starts to brown too quickly, loosely tent the pie with aluminum foil for the last 20 minutes of baking. Let cool for at least 4 hours before serving. (Can be made a day ahead. Cover pie and refrigerate.) Makes 10 servings.