Preheat oven to 375 F
Butter the inside of a square 8" baking dish
In a bowl, combine the oats, half the walnuts, baking powder, cinnamon and salt. In another bowl, whisk together the maple syrup, milk, egg, butter, and vanilla.
Spread the chopped apple pieces over the bottom of the buttered pan. Top with the oat mixture and drizzle the liquid mixture over the oats. Scatter the walnuts and berries over the oats.
Bake for 35-45 minutes, until the top is golden and the oats are set.