Add the flour to a large bowl. Remove 1 Tbsp of the flour and return it to the flour container. Add in the cornstarch, baking soda, salt, and cinnamon. Mix until combined.
In another bowl, mix together the egg, Greek yogurt, canola oil, vanilla extract, and brown sugar. Mix until well combined. Fold the chopped apple into the batter.
Fill each cavity of the muffin tin until about ¾th full.
In a small bowl, toss together the flour, quick oats, cinnamon, and brown sugar. Stir in the canola oil. The mixture will be wet. Top each muffin evenly with this topping.
Bake for 20-25 minutes or until a tester comes out clean when inserted in the center of the muffin.
Allow to cool for a few minutes before removing the muffins from the tin.
Notes
If you only have old-fashioned oats, add oats to a mini chopper or give them a quick chop with a knife to break them down into smaller pieces.