Preheat oven to 400F/205C.
In a medium bowl, whisk together the flour, sugar, salt, baking powder, cinnamon, and nutmeg.
In a large bowl, whisk together the egg, oil, milk, and vanilla extract.
Then add the flour mixture and mix, using a silicone or wooden spoon. If it is thick, add 1 to 2 additional Tbsp of milk.
Rinse and dry your apple, peel it or not. Next, chop it up into little chunks; the smaller the better.
Add those apple chunks and fold them into the batter.
Make the crumble topping. Mix together the softened butter, sugar, flour, and cinnamon in a medium bowl with a fork to make small to large clumps.
Add cupcake/muffin liners to your muffin pan and fill each one about 3/4 of the way full. This will make 12 muffins.
Sprinkle the crumble topping on top of each muffin.
Bake at 400F/205C for 25 to 28 minutes until golden brown on top and a toothpick comes out clean when inserted in the middle of the muffin.
Allow the muffins to cool in the pan for about 5 minutes, then transfer them to a wire rack to cool completely. Enjoy!