Tart apples and sweet blueberries, topped with a sugar oat topped and baked to perfection.
Preheat oven to 350°F and grease a 8x10 inch or 9x9 inch baking dish. Set aside.
In a large bowl, add the sliced apples, blueberries, lemon juice, ground cinnamon, corn starch, granulated sugar, and vanilla extract; mix until everything is combined. Pour this mixture into the greased baking dish and spread evenly.
In a medium bowl, mix together brown sugar, oats, all-purpose flour, ground cinnamon, and salt. With a pastry cutter or fork, cut the butter into the mixture until well combined. There should be small clumps throughout. Sprinkle this mixture evenly over top the apples and blueberries.
Bake in preheated oven for 45-55 minutes, until top golden brown and bubbling around edges.
Remove from oven and let rest for 10 minutes. Serve warm with a scoop of vanilla ice cream or whipped cream, if desired.