Preheat oven to 190 degrees C. Line a muffin tray with muffin cases.
Mix the flour, sugar, baking powder, bicarb, spices and salt.
In a bowl whisk the eggs. Stir in the sour cream and butter.
Stir the diced apple into the flour mixture. Add the egg mixture. Mix everything well with a spatula until just moistened (you may still see a few dry spots). Pour the batter into the prepared muffin cases.
For the crumble topping rub all the ingredients together with your fingertips and sprinkle over the muffins. Bake for 25 minutes in the preheated oven or until a skewer comes out clean.
Cool the muffins on wire cake rack and cool completely before serving.