In a large mixing bowl (or directly in the casserole dish), mix the wet ingredients and dry ingredients together in one bowl: bananas, rolled oats, maple syrup, Greek yogurt, eggs, milk, vanilla, cinnamon, salt and baking powder. Combine the oat mixture well (I like to use a whisk).
Next, pour the oatmeal mixture into a non-stick or greased casserole dish.
Combine brown sugar and cinnamon in a small bowl and sprinkle evenly on top of the oatmeal mixture.
Bake oatmeal in the oven for about 30-35 minutes until golden brown and firm in the middle.
To make the glaze, combine powdered sugar and milk. Drizzle on top of the cinnamon roll baked oats and serve warm. Store leftovers in an airtight container for up to 5 days.