2/3 cup walnuts
1/2 cup (packed) golden brown sugar
1/4 cup yellow cornmeal
1/4 cup all purpose flour
3/4 teaspoon ground cinnamon
1/2 teaspoon ground nutmeg
5 tablespoons chilled unsalted butter, cut into small pieces
2 1/4 pounds Granny Smith apples (about 6 medium), peeled, quartered, cored, cut into 1/2-inch-thick wedges
1/2 cup sour cream
1/4 cup sugar
2 tablespoons all purpose flour
1 1/2 teaspoons ground cinnamon
3/4 teaspoon ground nutmeg
1/4 teaspoon ground cloves
For streusel: Combine nuts, brown sugar, cornmeal, flour, cinnamon and nutmeg in processor. Using on/off turns, process until nuts are finely chopped. Add butter and process until small moist clumps form. (Can be prepared 1 day ahead. Cover and refrigerate.)
For filling: Position rack in center of oven and preheat to 375°F. Toss apples with sour cream in large bowl to coat. Mix sugar, flour, cinnamon, nutmeg and cloves in small bowl. Sprinkle mixture over apples and toss to coat.
Transfer filling to greased casserole dish. Sprinkle streusel over apples, covering completely. Bake pie until apples are tender and streusel is golden, tenting with foil if streusel browns too quickly, about 1 hour. Transfer to rack and cool slightly.
Serve slightly warm or at room temperature with vanilla ice cream or whipped cream.