Preheat your oven to 350°F (175°C) and line a muffin tin with paper liners or grease it lightly.
In a large mixing bowl, whisk together the flour, granulated sugar, baking powder, baking soda, salt, cinnamon, and nutmeg.
In another bowl, combine the melted butter, egg, and buttermilk. Mix well until fully combined.
Pour the wet ingredients into the dry ingredients and stir until just combined. Gently fold in the diced apples.
In a separate bowl, prepare the crumb topping by mixing together the flour, brown sugar, softened butter, and cinnamon until crumbly.
Spoon the muffin batter into the prepared muffin tin, filling each cup about 2/3 full. Sprinkle the crumb topping generously over each muffin.
Bake in the preheated oven for 18-22 minutes, or until a toothpick inserted into the center comes out clean.
Allow the muffins to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.