Preheat oven to 350 F. Line an 8×8 pan with parchment. In a large bowl, stir together the oats, flour, cinnamon, salt, baking powder, and sugar. Stir in the oil, then very firmly press 2/3 of the dough into the prepared pan, reserving the remaining dough. In a separate bowl, stir berries, starch, and 1 tbsp sweetener. Press this on top of the dough in the pan, then sprinkle the reserved dough on top. Press down very firmly again. Bake 35 minutes on the center rack. Let cool, then refrigerate at least 1 hour before cutting, as they firm up. Eat plain, or top with melted coconut butter frosting, or a basic powdered sugar glaze.