Preheat oven to 350 degrees. Grease or spray 12 muffin tin
In a high powered blender, add all ingredients except the optional topping. Make sure to put the apple, applesauce, yogurt, and eggs at the bottom and the oats at the top.
Blender well. Use the tamper to mix, if your blender has one. If not, stop the blender - scrape down the sides with a spatula - blend. And, repeat as needed until it is well combined.
Pour into pan. Each muffin tin will be about 2/3rd's of the way full.
In a small bowl, mix the topping - sugar, oats, and cinnamon - and evenly distribute on top of the muffins before baking.
Bake for 16 - 20 minutes or until firm, slightly brown, and a toothpick inserted into the middle comes out clean.
Allow to cool slightly before removing from the tin. Cool completely on a cooling rack before storing. Store on the counter for 2 - 3 days, in the refrigerator for 7 - 10 days, or the freezer for a couple months.