Preheat the oven to 350°F (175°C). Lightly grease an 8-inch or 9-inch square baking dish.
In a small bowl, combine the ground flax meal and warm water. Let the mixture sit for 3–5 minutes until it thickens.
In a large mixing bowl, stir together the rolled oats, baking powder, salt, cinnamon, and vanilla powder.
In another bowl, whisk together the flax mixture, maple syrup, almond milk, and oil.
Pour the wet ingredients into the oat mixture and stir until evenly combined.
Gently fold in the mixed berries.
Transfer the Vanilla Berry Baked Oatmeal mixture to the prepared baking dish and spread evenly.
Top with a few extra berries.
Bake for 15 minutes.
Remove from the oven and sprinkle the sliced almonds or chopped pecans along with the brown sugar over the top.
Return to the oven and bake for another 10 minutes, or until the oats are set and most of the liquid has been absorbed.
Allow the baked oatmeal to cool slightly before slicing.
Serve warm or cold with extra milk, maple syrup, jam, or nut butter if desired.