Preheat your oven to 375°F (190°C) and line a muffin tin with paper liners or lightly grease it.
In a large bowl, whisk together the flour, granulated sugar, baking powder, baking soda, ground cinnamon, and salt.
In another bowl, mix the melted butter, buttermilk, and eggs until combined.
Gradually add the wet ingredients to the dry ingredients, stirring gently until just combined. Do not overmix.
Fold in the diced apples until they are evenly distributed throughout the batter.
In a separate bowl, prepare the crumble topping by mixing brown sugar, rolled oats, chopped nuts, 2 tablespoons flour, and cinnamon together until crumbly.
Spoon the muffin batter into the prepared muffin tins, filling each cup about two-thirds full.
Sprinkle the crumble topping generously over each muffin.
Bake in the preheated oven for 18-20 minutes or until a toothpick inserted into the center comes out clean.
Remove from the oven and let the muffins cool in the tin for a few minutes before transferring to a wire rack to cool completely.
Notes
For a richer flavor, use a mix of different apple varieties.