To make the topping:In a medium sized bowl, combine the flour, oats, sugars, cinnamon, and salt. Stir to combine.

Pour the melted butter over the dry ingredients and mix until evenly moistened and clumps start to form. Set aside.

To make the apple blueberry crisp filling:Preheat your oven to 350º and spray an 8x8" baking dish with cooking spray or grease with softened butter.

Lower your oven rack to one level below the middle position. 

In a large bowl, combine your sliced apples and blueberries. 

Pour the lemon juice over the fruit and sprinkle with the sugar, flour, and cinnamon. Use a rubber spatula to gently toss everything together until evenly coated.

Pour the fruit into your prepared baking dish. Depending on how deep your baking dish is, the fruit may come above the top. That’s ok, the fruit will cook down during baking. 

Using your hands, crumble the topping over the blueberries and apples, making sure to keep any large chunks intact.

Place the apple blueberry crisp on a rimmed sheet pan (in case it bubbles over) and bake in a 350º degree oven for 60-70 minutes.

Bake until the topping is brown and crispy and the juices are thick and bubbly in the corners. You can test to see if the apples are cooked by sticking a small knife in the center of the crisp. If there is no resistance when pulled out, the crisp is done!

Let cool for 30 minutes before serving.