Preheat the oven to 375 degrees F. In a mixing bowl combine all fruit filling ingredients and mix to combine. Set aside and allow to rest for at least 5 minutes so the juices can release.
In another mixing bowl combine remaining all-purpose flour, brown sugar, white granulated sugar, rolled oats and a pinch of salt. Stir to combine. Add in the cubed, cold unsalted butter. Use a pastry cutter (see about for alternative methods) and/or your hands to press or pinch the butter into the flour mixture until it is well incorporated and forms a crumbly texture.
Prepare the bottom of a 10 inch casserole dish or cast iron skillet with a little butter. Stir the fruit mixture to incorporate the juices and pour into the baking dish. Sprinkle the flour and butter mixture evenly over the top of the fruit. Bake at 375 degrees F for about 40 minutes until the fruit is bubbly and the topping is crisp golden brown. Remove from the oven and allow to set for at least 5-10 minutes before serving. Serve hot with ice cream, custard, whipped cream, or simply on its own. Enjoy!
Notes
This recipe is great to make ahead. Simply refrigerate and heat at 375 for about 15 minutes before serving.