Preheat oven to 175°C / 350°F (top & bottom heat).
Mix all filling ingredients in a bowl and spread into a pie dish.
In another bowl, combine sugar, salt, oats, flour, and baking powder.
Melt the butter, then mix it with the syrup and stir into the dry ingredients until smooth.
Spoon the crumble over the blueberries and gently spread evenly.
Bake in the middle of the oven for 30–40 minutes, until golden.
