Preheat oven to 350°F (177°C). Lightly grease a 9×13-inch baking pan.

Mix all of the filling ingredients together in a large bowl, then spread into the baking pan.

Whisk the flour, brown sugar, and cinnamon together in a medium bowl. Cut in the butter using a pastry cutter or fork until the mixture becomes super crumbly. Stir in the oats. Sprinkle over filling.

Bake for 40-45 minutes or until the topping is golden brown and the fruit juices are bubbling around the edges. Remove from the oven, place on a wire rack, and allow to cool for at least 5 minutes before serving. Serve warm, room temperature, or cold.

Cover leftovers and store in the refrigerator for up to 5 days.