To make walnut crumb topping, combine 3/4 cup walnuts, 1/4 cup granulated sugar and 1/4 cup packed brown sugar in a food processor. Cover and process to grind the walnuts. Remove the lid and add 1/2 cup all-purpose flour, 1/2 cup rolled oats, 1/4 teaspoon salt and 1/4 teaspoon cinnamon. Cover and process until combined. Remove the lid; scatter 6 tablespoons cold butter, cut into 1/4-inch-thick-slices, over the flour mixture. Cover and process until mixture resembles coarse crumbs. Transfer to a large bowl; rub the crumbs between your fingers to make large, buttery crumbs.