In a large bowl, combine the blueberries, apples, sugar, cornstarch and salt. Stir well to combine and allow to sit, stirring occasionally.
Meanwhile, in another bowl, combine the ingredients for the sugar cookies and use a fork or preferably, a pastry cutter. It should resemble a coarse meal.
Take 2/3 of the cookie dough and press into a tart pan. If you have one with a removable bottom, it will release cleaner but you can also line with parchment.
Add the fruit to the tart dough. Do not overfill as the cookie base will expand during baking.
Take the remaining crumbled dough and liberally sprinkle over top.
Bake at 350 degrees for 30-40 minutes or until the top is golden brown. The baking time will vary based on your oven and choice of pan, so keep a close eye.
Allow to cool slightly prior to serving.