Preheat the oven to 375 degrees. Spray a 9x9 inch baking pan with cooking spray. Start by toasting the walnuts until fragrant, about 4-5 minutes. Remove walnuts from pan and roughly chop; set aside.
In a medium bowl, mix together oats, walnuts, cinnamon, nutmeg, baking powder and salt.
In a small bowl, whisk together milk, maple syrup, eggs, butter and vanilla. Pour mixture into the bowl with the oats. Stir in mixed berries then spread into even layer in baking dish.
Bake for 40-45 minutes or until golden brown on top. Serve with a dollop of Greek yogurt if you wish.
Notes
Can be served warm, room temperature or chilled.Recipe adapted from Cookie and Kate