For the crumble: Mix all ingredients with your hands until crumbly and refrigerate.
For the apple filling: Mix apples, butter, brown sugar, cornstarch, cinnamon and nutmeg in a pan over medium-low heat. Stir frequently until butter is melted and sugar is dissolved. Simmer for about 5 minutes until apples are soft but still chunky. Let cool.
For the batter: Mix flour, baking powder, cinnamon and salt in a bowl. Beat butter, oil and both sugars until creamy for 2-3 minutes. Stir in eggs and vanilla. Alternately stir in milk and yogurt with the flour mixture. Stir only until a smooth batter forms. Finally fold in the apples.
Fill muffin cups two-thirds full with batter. Add 1-2 teaspoons of apple filling and generously sprinkle with crumble topping.
Bake at 220°C (top and bottom heat) for 5 minutes, then reduce temperature to 170°C and bake for another 18-20 minutes until golden brown.
