This sweet peaches and blueberry crisp can be made with fresh or frozen peaches and is delicious served warm with a scoop of vanilla ice cream over the top.
Preheat the oven to 350°F. Rub softened butter all around the inside of a 8x11-inch baking dish and set aside.
Peel the peaches and slice them. Add them to a large mixing bowl with the fresh blueberries. Add the sugar, brown sugar, cinnamon, vanilla, lemon juice, and cornstarch to the bowl and gently toss the fruit in the sugars with a spatula until everything is well combined.
Pour the fruit filling into the prepared baking dish. Be sure to scrape all the juices from the bowl.
To make the crisp topping, add the flour, sugar, brown sugar, kosher salt, and oatmeal to the bowl of a stand mixer fitted with a paddle attachment. Stir them together until well combined.
Cut the cold butter into small diced pieces and add it to the mixer. Beat the butter into the sugar mixture until the flour and oats are well coated in butter and everything starts to stick together in small pea-sized clumps.
Sprinkle the prepared crisp topping over the blueberry peach filling. You may want to place the baking dish on a baking sheet in case any of the fruit juices spill over during baking.
Bake the fruit crisp for 45 minutes or until the topping is golden brown and the fruit filling is hot and bubbly. Serve warm with vanilla ice cream on top.