Preheat oven to 350ºF. Prepare a baking dish (I use 8” x 12” glass dish, see note) by wiping the inside with a drop of oil and lining with a strip of parchment paper.
In a large bowl, add oat flour, sugar, zest, baking powder, baking soda, and salt.
Stir through to combine.
In another bowl, add milk, vinegar, and vanilla, stir through.
Add wet ingredients to the dry, and mix through to incorporate.
Finally, add the berries and gently but quickly mix through.
Pour batter evenly into prepared pan.
Sprinkle on the 2 tbsp of reserved sugar and rolled oats.
Bake for 25-28 minutes, until lightly golden and a toothpick inserted in the center comes out clean.
Remove from oven and cool in pan before cutting into pieces. Serves 6 or more.