For Topping: 1 cup all purpose flour 1 cup walnuts 2/3 cup (packed) golden brown sugar 1/2 cup (1 stick) chilled unsalted butter, cut into small pieces
Make Topping: Combine all ingredients in processor. Process until walnuts are coarsely chopped and small moist clumps form. Transfer topping to medium bowl. Cover and refrigerate until firm, about 1 hour.
For Pears: 3 1/2 pounds firm but ripe Anjou pears, peeled, cored, cut into 1/2-inch pieces 2/3 cup pure maple syrup 1/2 cup raisins 2 tablespoons all purpose flour 2 tablespoons fresh lemon juice 1 tablespoon finely chopped crystallized ginger Sour cream
Make Pears: Position rack in center of oven and preheat to 350°F. Toss all ingredients except sour cream in large bowl to blend. Let stand 15 minutes. Transfer pear mixture to 13 x 9 x 2-inch baking dish. Sprinkle topping over pears. Bake until pears are tender, juices bubble thickly and topping is golden and crisp, about 30 minutes. Let stand at least 10 minutes. Serve warm with sour cream.
