1 stick (1/2 cup) salted butter, cold, cut into small cubes
Preheat oven to 350 degrees with the oven in the middle position.
In a mixing bowl, make the berry filling by combining all ingredients and tossing to combine. Set aside.
In a separate mixing bowl, make the oat crumble by combining all ingredients, then use your fingers to pinch the butter and incorporate with the dry ingredients. Keep pinching and tossing together with your hands until evenly combined. You will be left with many pea sized pieces of butter mixed in with the oat mixture. Set aside.
Pour the berry filing into a 1-1/2 quart baking dish. Top with the oat crumble topping, and spread to an even layer.
Bake uncovered until the berry filling is bubbling, and the oat crumble is golden brown, about 1 hour. Let cool for 1 hour before serving. Serve room temperature with ice cream. Will keep covered at room temperature for 3 days