Preheat the oven to 375 degrees F. Grease a 12-cup muffin tin and set aside.
Into a large bowl, sift together the flour, baking powder, baking soda, salt, cinnamon, and nutmeg. In another bowl, whisk together the eggs, buttermilk, and melted butter. Add the sugar and whisk to combine. Add to the dry ingredients and mix until just combined. Fold in the apples and walnuts. Divide the batter among the muffin tin cups, filling each about 2/3 full.
To make the streusel topping, in a small bowl, combine all the ingredients and stir until well combined and crumbly.
Sprinkle about 1 tablespoon of the crumb mixture over the top of each muffin. Bake until a tester inserted into a muffin comes out clean, about 25 minutes. Remove from the oven and let sit in the muffin tin for 10 minutes. Turn out onto a wire rack and let cool slightly before serving.
Yield: 12 muffins