Preheat oven to 350°. Coat a 9 x 13 baking dish with nonstick cooking spray.
In a large mixing bowl use a whisk to combine lemon juice, vanilla extract, and cornstarch. Add peaches, blueberries, and sugar and toss the mixture lightly with a spoon until well combined. Pour into a baking dish.
In a medium mixing bowl beat butter with brown sugar until snooze and creamy. Add flour, oats, and salt & stir to combine. Use your hands to need the mixture until it is crumbly. Sprinkle the crumbled mixture evenly over the peaches and berries.
Bake 350 to 60 minutes till browned and bubbly. Allowed a cool for 15 to 20 minutes before serving. Great served warm with vanilla ice cream.