Preheat the oven to 375 degrees F. Grease an 8x8 baking dish and set aside.
Cut the peaches into thin slices. I like to leave the skin on, but you can peel them if you want. Peel the banana and cut into slices. Make a single layer of peach and banana slices on the bottom of the greased baking dish, saving about half of the peach slices for later.
Combine the oats, cinnamon, baking powder, salt, and about half of the walnuts in a mixing bowl. Spread this mixture on top of the fruit slices in the baking dish.
In the mixing bowl, whisk together the milk of choice, maple syrup, half of the melted butter, egg, and vanilla extract. Pour this over the dry ingredients and fruit. Gently shake the dish if needed to spread the liquid out evenly.
Place the rest of the peach slices on top, followed by the rest of the walnuts.
Bake for 35 to 45 minutes until lightly browned. Remove from oven and drizzle with the rest of the melted butter. Serve with maple syrup if desired.
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