Pre-heat the oven to 350 degrees. Lightly mist a 12-count standard-sized muffin tin with cooking spray and set aside.
Place the pineapple chunks in a bowl and add one tablespoon of the brown sugar. Stir to coat the pineapple. Place half of a maraschino cherry in the center of each well of the muffin tin and surround it with the coated pineapple chunks. If there is liquid remaining in the bowl from the pineapples releasing juice/mixing with the sugar, save it and add it with the wet ingredients in step four.
In a large mixing bowl, add the oats, the remaining ¼ cup of brown sugar, the baking powder, and the salt and stir together to combine.
In a separate mixing bowl, add the egg, egg white, milk, applesauce, and vanilla extract and whisk together until combined. Add the wet ingredients to the dry ingredients and stir together until combined.
Spoon the batter evenly amongst the wells in the prepared muffin tin over top of the cherries and pineapple chunks. Bake for 20-24 minutes until the oatmeal is lightly browned and a toothpick inserted into the center comes out clean. Allow them to cool in the tin for at least 10 minutes before removing and flipping each oatmeal single upside down (so the pineapples are on top) for serving.