Add the 1.5 cups milk, ⅓ cup plain yogurt, ½ cup maple syrup, 2 eggs, and 1 teaspoon vanilla extract to a large mixing bowl. Whisk until well combined.
In another bowl, stir together the 3 cups rolled oats, 1 teaspoon baking powder, ¼ teaspoon salt, and 1 teaspoon cinnamon.
Pour the wet mixture into the dry ingredients and stir until just combined. Fold in the grated apple.
Lightly grease a 9x13 inch baking pan. Pour the batter in and spread evenly.
Bake on the middle rack for 30-35 minutes, until the top is set and lightly golden.
Remove from the oven and allow to cool for 5-10 minutes before cutting into squares. Serve warm or at room temperature.
Notes
This recipe will not work with quick cooking oats, steel cut oats, or instant oatmeal.
Grease the baking dish first. Lightly grease the 9x13-inch baking dish before adding the oatmeal mixture to help keep it from sticking.