To prepare streusel: In a large mixing bowl, combine flour, oats, brown sugar and cinnamon and rub together with your fingertips to blend and break up any clumps of brown sugar. Scatter butter cubes over flour mixture and continue to rub ingredients together with your fingertips until it forms small clumps and butter is no longer visible. Stir in walnuts and reserve.
To prepare brown betty: Preheat oven to 375 degrees. In a large bowl, combine sugar, flour, cinnamon, cardamom and nutmeg. Add apples and toss gently to coat with flour mixture. Arrange half of apple slices in an even layer in an 8-inch baking dish or other shallow 1 1/2-quart glass or ceramic casserole. Sprinkle half of gingersnaps over apples. Repeat with a remaining layer of apples and gingersnaps. Drizzle cream over gingersnaps and top with an even layer of streusel. Bake until apples are tender, 45 to 50 minutes. Remove from oven, transfer baking dish to a wire rack and cool until warm. Serve with a scoop of ice cream, if desired.