Preheat the oven to 350 degrees F and line an 8x8 baking dish with parchment paper.
Make the crumble by mixing the oats, walnuts or pecans, almond flour, brown sugar, flaxseed, cinnamon and salt in a food processor. Once combined, add the coconut oil and pulse again. Then add the water and pulse one more time until everything is mixed well.
Press 2/3 of the crumble into the baking dish to create the crust layer. Bake for 20-25 minutes and then remove from the oven and allow to cool for 10 - 15 minutes.
While the crust is baking, mix up your filling. Add the blueberries, brown sugar, lemon juice and cornstarch to a sauce pan over low to medium heat and stir occasionally until berries burst and the ingredients turn into a sauce type filling. Remove from heat and allow to thicken slightly.
Pour the blueberry filling over the slightly cooled crust and sprinkle with remaining crumble or as much of the crumble as your prefer. Garnish with a few more oats and return to the oven to bake another 20 minutes, until the crumble topping is crisp.
Let cool completely before slicing into bars and serve immediately or store cooled bars in a tupperware container in the fridge. Enjoy with ice cream or on their own!