Preheat oven to 350°F.
Combine the oats, milk, egg, sugar, butter or oil, vanilla, cinnamon, baking powder and salt in a medium size bowl. Stir until combined.
Add 1 ½ cup frozen berries to the bottom of a 10x5" or 8x8" pan. Then pour the oat mixture over top. Add ½ cup remaining berries on top. Bake for 25 minutes (needed 20 extra mins in unpre-heated pan), or until the edges are slighty crisp and the middle is set. Remove from oven and allow to cool 10 minutes before serving.
Serve warm with Greek yogurt or desired toppings. Store leftovers in a container and refrigerate for up to 1 week.
