Preheat the oven to 350 and prepare an 8x8 pan with parchment and grease. Set to the side.
In a large bowl, whisk together the flour, oats, brown sugar, baking powder, lemon zest, and salt. Add in melted butter and mix to combine.
Add about ⅔ of the crust mixture to the bottom of the prepared pan. Use the back of a spoon or a rubber spatula to flatten out the mixture.
Bake the crust in the oven for 10 minutes.
While the crust is baking, make the berry filling. Roughly chop the strawberries and add the blueberries and raspberries whole into a saucepan. Add the sugar, cornstarch, lemon juice, and zest into the saucepan and combine. Cook the mixture on medium-low heat for 2-3 minutes.
Take the crust out of the oven and pour the fruit mixture over top. Use the remaining crust as a topping, pouring it over the fruit topping to cover the majority of the filling.
Place the pie bars in the oven and bake for 45-55 minutes. When they are ready, the fruit will bubble from the bars.
Let the bars fully cool in the pan before slicing. This ensures clean slices.
