Preheat oven to 375 degrees F. Grease an 8" square baking pan with butter, or spray with oil.
In a large bowl, combine oats, walnuts, pumpkin pie spice, baking powder, and sea salt.
In a separate bowl, combine milk, maple syrup, egg, vanilla, and melted butter.
Line the bottom of the baking dish with sliced plums. Spoon the dry oat mixture on top of plums. Pour milk mixture on top, and make sure it seeps through. Top with another layer of sliced plums, and extra chopped walnuts and oats as desired.
Baked for 35 to 40 minutes or until plums are caramelized and tender on top. Enjoy!