1 Preheat the oven to 350°F (175°C). Line an 8×8-inch (20×20 cm) baking pan with parchment paper, leaving an overhang for easy removal.

2 In a large bowl combine the rolled oats, flour, brown sugar, granulated sugar, salt, baking powder, and cinnamon. Mix until evenly distributed.

3 In a separate bowl whisk together the melted butter, beaten egg and vanilla extract. Pour the wet ingredients into the dry ingredients and stir until a crumbly dough forms.

4 Reserve about 1 cup of the oat mixture for the topping and press the remaining mixture firmly and evenly into the bottom of the prepared pan to form the crust.

5 Prepare the blueberry filling: in a medium bowl toss the blueberries with honey (or maple syrup), cornstarch and lemon zest until coated.

6 Spread the blueberry mixture evenly over the pressed oat crust, taking care not to overload any one spot.

7 Crumble the reserved oat mixture over the blueberry layer to create a uniform topping.

8 Bake for 30–35 minutes, or until the topping is golden brown and the filling is bubbling at the edges.

9 Remove from the oven and let cool completely in the pan on a wire rack (at least 1 hour) so the filling sets.

10 Use the parchment overhang to lift the bars from the pan. Cut into 12 bars and serve. Store leftovers in an airtight container in the refrigerator for up to 4 days.