Begin by making the filling. Over medium heat, bring the rhubarb, raspberries, and sugar to a boil. Let the mixture cook until mostly all the rhubarb has lost it's shape, about 15 minutes.
Take the fruit mixture off the heat and stir in the orange zest, vanilla, and chia seeds.
While the filling is cooling (about 15 minutes), preheat the oven to 350 degrees Fahrenheit and line a 9 x 9 inch pan with parchment paper.
Mix together the flour, oats, brown sugar, and salt in a medium sized mixing bowl.
Add the melted butter to the flour and oat mixture and mix well.
Take 2/3 of the crumble and press it down into the prepared pan. Then spread the fruit mixture on top. Finally, place the remaining crumble on top of the filling.
Bake in the oven for 55 minutes. Let cool completely (yes, I am talking a few hours) on a wire rack before cutting into slices.
* I've tried a few different variations, including rhubarb + raspberries; strawberries + blueberries; and (as photographed) raspberries + blueberries. Essentially, I use whatever fruit I have in the fridge!