Preheat oven to 350 degrees. Core apples with a melon baller, making a 3/4-inch-wide cavity, stopping 1/2 inch from the bottom. Peel top inch of apples; fill with cherries and walnuts. Arrange in an 8-inch square baking dish.
Drizzle maple syrup over apples; top each with 1/2 tablespoon butter. Bake, brushing occasionally with accumulated juices, until apples are tender, 45 to 60 minutes.
Transfer apples to a plate. Pour liquid from dish into a bowl; whisk in sour cream. Drizzle over apples; sprinkle with gingersnaps.