Make the Streusel

1. In a large bowl add all of the streusel ingredients EXCEPT the butter and mix to combine. Add in the very soft butter and with either clean hands or a fork, press the butter into the oat mixture until it forms clumps and coated crumbs; set aside. *This can be made 1 day in advance. Simply make, place in an air-tight container, and refrigerate for up to 3 days.

Make the Fruit Filling

1. In a small bowl, whisk together the sugar, cornstarch, and cinnamon. In a large bowl add the berries and lemon juice. Gently stir to combine. Add the sugar mixture to the fruit and stir to coat all of the pieces. Set aside while the oven preheats.

Assemble & Bake

1. Preheat the oven to 350F. In two 3-quart baking dishes or one 9×13″ (13×9″) baking dish, add the fruit mixture and all of the liquid from the bowl.
2. Next, add the streusel mixture to the fruit and press down slightly.
3. Place the dish on a rimmed baking sheet and put it in the oven. Bake for 30-45 minutes or until the fruit pierces easily with the tip of a thin blade and the streusel is golden brown.
4. Remove from the oven and place the pan on a wire rack. Allow to cool for at least 20-25 minutes before serving.
5. Store the leftovers in the pan, covered and in the fridge for up to a week.