Preheat the oven to 180c/350f. Grease and line a square baking tin. Tip: leave "handles" of the baking paper hanging out of the tin to make it easy to lift the squares out!
In a large bowl, mix the oats, flour, cinnamon and a pinch of salt.
Mix in the coconut oil and honey, mixing well until even crumbles form. The mixture should be crumbly, but should easily press and stick together.
Press the oat mixture in to the bottom of the tin, leaving around 1/3 of a cup to sprinkle on top of the bars. Use a spoon or spatula to press it evenly in to the tin.
In a small bowl, mix the blueberries, lemon and cornstarch. Sprinkle the berries over the oat base and sprinkle with the remaining oat mixture and 1 tbsp of brown sugar.
Bake for 30-40 minutes, until the blueberries are broken down and bubbling and the oat crumb is lightly golden. If you used frozen fruit, the time will be roughly 40 minutes, but fresh will be more like 30, check the berries!
Allow to cool completely in the tin before lifting out and cutting in to squares. Either cut in to 9 large (2x2 cuts) or 16 smaller (3x3 cuts) squares.