In a large bowl, combine the first eight ingredients. In another bowl, beat the eggs, sour cream and butter. Stir into dry ingredients just until moistened. Fold in apple. Fill greased or paper-lined muffin cups one-third full.
For topping, combine the walnuts, flour, sugar, cinnamon and nutmeg. Cut in butter until mixture resembles coarse crumbs. Sprinkle about two-thirds of the topping over batter. Top with remaining batter; sprinkle with remaining topping.
Bake at 375° for 20-25 minutes or until a toothpick inserted neat the center comes out clean. Cool for 5 minutes before removing from pans to wire racks. Serve warm.