Preheat oven to 375 degrees F. Grease an 8-inch square glass baking dish.
Combine oats, baking powder, cinnamon, and salt in large bowl. Whisk together yogurt, milk, maple syrup, egg and vanilla in another bowl; pour over oat mixture and stir to combine. Add chopped apples and mix until incorporated.
Pour mixture into prepared dish and spread out with spatula until evenly distributed.
Bake for 35 minutes or until edges are browned and knife inserted in centre comes out clean. Let baked oatmeal stand a few minutes before serving.
Store in the fridge for up to a week or in the freezer for up to a month.
Chop up the apple in small pieces that are uniform in size and make sure that the apple chunks are evenly distributed throughout the batter so that you get all of that apple goodness in each bite.
Grease the baking dish well to prevent the baked oatmeal from sticking to the pan. You can also line the pan with parchment paper for easy removal.
Allow the baked oatmeal to sit in the baking dish for at least an hour to cool. It needs time for the oats to absorb some of the extra moisture. If you try to take it out of the pan too soon, it may stick to the bottom.