Preheat the oven to 350 F/ 180 C/ gas mark 4. Lightly grease a 22 cm/8.5 inch pan and line with parchment paper. Set aside.
Place the oats, almond flour, flaxseed, coconut oil, maple syrup and the pinch of salt in a food processor.Pulse a few times until all the ingredients have been incorporated and the mixture is sticky. Do not overmix the ingredients.
Press ¾ of the oats mixture into the base of your pan.
In a mixing bowl combine the blueberries, lemon zest, sugar and cornstarch and stir thoroughly.Spread the mixture over the oat base.
Crumble the remaining oat mixture over the top, tuck into the blueberries a little by pressing gently and bake in the centre of the oven for 35 minutes.
Remove from the oven and set aside to cool completely before cutting.
Do not overprocess the ingredients. They should be mixed thoroughly but still have lots of texture.
Make ahead: You can prepare the oat mixture, press into the pan and refrigerate (overnight). Keep the rest of the mixture in the fridge until you are ready to assemble the recipe. I do not recommend preparing the blueberry mixture ahead.
I used a moderate amount of sugar but you can use more if you prefer (just taste the oat mixture and add more maple syrup if needed).
Cool completely before serving (they will cut better).
These blueberry oat bars are best served on the day they are made. Leftovers can be refrigerated (but the topping will lose its crunch) for up to 3 days.
Freeze (unbaked) for up to 3 months. Bake from frozen until golden/lightly browned (they will need a few minutes longer in the oven when cooking from frozen).