Preheat the oven to 190C/gas 5/fan 170C. Put the apples in a 1.5 litre pie dish. Pour over the fruit juice, then add the ginger, and 1 tbsp of its syrup and stir together.
Make the topping. Tip the flour into a large bowl. Add the fat and cut into small pieces using a round-bladed knife, so that it is evenly distributed through the flour. Stir in the muesli, sugar and sunflower seeds, then scatter evenly over the apples to cover them.
Bake for 40-45 minutes until golden and crunchy on top. Leave to cool slightly for 5-10 minutes before serving with reduced fat Greek yogurt or low-fat natural yogurt.