1. Preheat the oven to 350°F (180°C). Grease an 8 inches (20cm) springform tin lightly and line with some baking parchment. Set aside.

2. Add the (fresh or frozen) blueberries to a pot, add the cornstarch, maple syrup, and vanilla extract, stir and bring to a boil. Remove the heat and let cook for several minutes until the liquids thicken up and you get a very thick blueberry sauce. Take off the heat and let cool down a bit.

3. For the streusel add the flour, sugar, rolled oats, sliced almonds, baking powder, cinnamon, and salt to a large bowl and mix to combine. Melt the butter and add together with the vanilla extract to the bowl – mix with a fork until streusel form. Add about 2/3 of the streusel to the springform tin and press to the bottom to get one even layer. Pour the slightly cooled blueberry filling on top of the streusel base and sprinkle the remaining streusel on top. Bake for about 30-35 minutes or until the streusel got a nice golden color and the blueberry filling is bubbly. Take out of the oven and let cool down completely on a wire rack. When cooled, place in the fridge for at least 2 hours in the fridge. Cut into slices or bars before serving.