In a bowl, stir together the flour, oats, sugar, and cinnamon. Pour melted butter over the to and stir to combine.
For the Blueberry Filling
In a small bowl, combine the sugar, flour, cornstarch, lemon zest, and chia seeds. Add blueberries to a large bowl. Pour flour mixture over the top. Stir to combine.
Sprinkle with lemon juice. Toss gently to combine.
Spoon the blueberry filling mixture into a 9x9-inch baking dish or casserole dish. Top with crumble topping.
Place on a baking sheet and bake for 50 minutes until the sides are bubbly and the crumble is golden brown.
When done, remove it from the oven. Set it aside and allow it to cool for 3 hours at room temperature. This allows the filling to thicken properly.
If there any leftovers, cover and store them in the fridge for up to 5 days.
Notes
Make this into a Gluten-Free Crumble by using a 1:1 gluten free baking flour like Bob's Red Mill Baking Mix.