Place a rack in the lower third of your oven and preheat the oven to 350℉. Spray a 9x9-inch baking dish with nonstick spray.
In a large, microwave-safe bowl, melt the butter. Set aside to cool.
In a separate, large bowl, whisk together the oats, cinnamon, baking powder and salt.
To the bowl with the cooled butter, add the milk, egg, applesauce, maple syrup and vanilla. Whisk to smoothly combine. Add this mixture to the dry ingredients, and stir gently to combine. The mixture will be wet. Pour the batter into the prepared dish. Spread into an even layer, and sprinkle the pecans on top (turbinado sugar too, if using). Bake for 40 minutes, until the oatmeal is golden at the edges.
PREPARE THE CINNAMON SWIRL:
In a small bowl, melt the butter in the microwave. Whisk in the powdered sugar, milk and vanilla. The swirl should be thick, but pourable (add more powdered sugar or milk, as desired). When the oatmeal comes out of the oven, drizzle on the cinnamon swirl. Let cool for 5 minutes, then serve hot. Drizzle more milk on top, if desired.
Notes
To make this recipe gluten free, just be sure to use gluten free oats.