Preheat the oven to 400°.
In a medium bowl, combine flour, oats, baking powder, sugars, and zest.
Melt butter and stir in until the mix is uniformly clumpy, then place in the fridge for 10-15 minutes.
While the crumble gets cold, make the filling. In a large bowl, mix fruits, juice, sugar, cornstarch, and salt.
Pour the filling into a 9x13 casserole dish and top with the crumble mix.
Bake for 35 minutes until the topping is golden and the filling is bubbling, then let sit for 15-20 minutes.