Melt the butter or margarine in a medium nonstick skillet over medium-high heat. Saute the apple until just softened, about 3 minutes, and remove from heat. Stir in the raisins, walnuts, sugar, cinnamon, nutmeg, and ½ cup sour cream.
In a medium bowl beat the eggs, milk, maple syrup, and remaining ½ cup sour cream. In a buttered 10-inch pie plate or 9-inch square baking dish, layer half the bread; spread apple-sour cream mixture over the bread and then layer with remaining bread. Pour egg-milk mixture over. Cover and chill in refrigerator for 2 to 24 hours.
Preheat oven to 325 degrees F. Bake strata in preheated oven 50 to 60 minutes, or until set and lightly browned. Let stand 10 minutes before cutting.