Preheat oven to 375-degree F. Line a 9x13 inch sheet pan with parchment paper extending it a few inches over the sides. Grease the pan properly with butter and set aside.

In a large mixing bowl, combine whole wheat flour, oats, brown sugar, cinnamon powder, vanilla extract, melted butter and salt. Stir to mix together until it forms a crumble like texture. Divide the mixture and set 1 cup aside. Transfer the remaining to the prepared pan and press it gently with your hands or spatula to form an even flat base.

In a separate bowl, toss berries with lemon juice, corn flour and sugar. Spread it evenly over the crust. Top with remaining crumble mixture and slivered almonds.

Bake for 45-50 minutes or until the crust is golden brown and the fruit mix is bubbly. Remove from the oven and allow it cool in room temperature. Refrigerate for 1-2 hour before cutting into squares.

You can refrigerate and store it in an air tight container for 3-4 days.