Preheat the oven to 350°F (175°C) and grease an 8x8-inch baking dish.
In a large bowl, stir together the oats, baking powder, cinnamon, and salt.
In another bowl, whisk the mashed banana, milk, egg, maple syrup, and vanilla until combined.
Pour the wet ingredients into the dry and stir. Gently fold in most of the strawberries, saving a handful for the top.
Pour into the baking dish and scatter the reserved strawberries over the surface.
Bake for 33-38 minutes, until set in the center and golden at the edges. Let cool for a few minutes before serving.