First, preheat your oven to 375°F and lightly grease a 9x9 baking dish with coconut oil or vegan butter. When the oven is ready, toast the chopped pecans on a baking sheet for 3-4 minutes (careful not to burn them).
In the meantime, mix 2 tbsp of ground flaxseed with 6 tbsp of water in a small bowl. Set aside. Then, in a large mixing bowl, whisk together rolled oats, baking powder, cinnamon, salt, and sugar.
In a third bowl, melt vegan butter or coconut oil, and add this along with the soy milk, flaxseed mixture, mashed banana, toasted pecans, and vanilla. Stir until combined.
Next, cover the bottom of your baking dish generously with frozen berries, about 1 ½ cups. Layer the oatmeal mixture over the bottom berry layer, then place the remaining berries evenly across the top of the oatmeal.
Add a sprinkle of cane sugar (optional) over everything before baking. Bake at 375°F for 40-45 minutes, or until the top is slightly spongy. Let it rest for 10-15 minutes, then serve warm with maple syrup, extra soy milk, or melted vegan butter on top.