Preheat oven to 350 degrees. Spray and 8x8 glass baking dish with cooking spray.
In a large bowl, mix together oats, cinnamon, baking powder and salt.
In a medium bowl, whisk together the milk, maple syrup, melted butter, egg, and vanilla. (Make sure the butter is cooled. I also like to add it before the egg, so the milk continues to cool it. Otherwise you can get bits of scrambled egg in your oatmeal...)
Pour wet ingredients over the dry ingredients and stir to combine. Gently stir in the chopped strawberries.
Pour mixture into baking ban and place the reserved sliced strawberries on top.
Bake for 40 minutes, until set.