For the Crumb Topping:

1. Get a small bowl ready. Put the flour, sugar, and ground cinnamon into this bowl. Use a whisk to mix them together.
2. Slowly pour the melted butter into the flour mix. Stir it gently as you pour. Keep mixing until the mixture looks crumbly with bits. Try not to stir too much or it will turn into a dough.
3. Place the bowl of crumble topping into the refrigerator. Let it chill while you make the muffin batter.

For the Muffins:

1. Preheat your oven to 350 degrees Fahrenheit (175 degrees Celsius). Place paper liners in a 16-cup muffin pan.
2. Take a large bowl. Whisk together the all-purpose flour, baking powder, baking soda, salt, and 2 teaspoons of the ground cinnamon. Set this bowl aside for now.
3. In a separate bowl, or the bowl of a stand mixer fitted with the paddle attachment, combine the white sugar and light brown sugar with the neutral oil. Mix until they are well combined.
4. Add the eggs one at a time to the sugar and oil mixture. Make sure to scrape down the sides of the bowl with a spatula whenever needed to get everything mixed in evenly.
5. Pour in the sourdough discard, milk, and vanilla extract. Mix these ingredients together. Don’t worry if the mixture looks a little curdled at this point. It is normal.
6. Add the dry ingredients (the flour mixture) to the wet ingredients. Mix on a low speed just until the dry ingredients are mixed into the wet ones. Stop mixing as soon as you don’t see any dry flour spots.
7. Peel and cut your apples into small pieces. Get a small bowl ready. Put the diced apples, the remaining 2 teaspoons of flour (from the divided amount), and the remaining 1/2 teaspoon of cinnamon into this bowl. Gently mix the apples until they are coated. This flour coating helps prevent the apple pieces from sinking to the bottom of the muffins during baking.
8. Add the coated apple pieces to the muffin batter. Carefully fold them into the batter using a spatula. Be gentle so you don’t overmix the batter.

Bake:

1. Divide the muffin batter evenly among the 16 lined muffin cups. Fill each cup about three-quarters full.
2. Take the chilled crumb topping from the fridge. Sprinkle a good portion of the crumb topping over the top of the batter in each muffin cup.
3. Place the muffin pan in the preheated oven. Bake the Sourdough Apple Cinnamon Muffins for 17 to 18 minutes. You can tell the muffins are done when the tops look golden brown and a toothpick inserted into the center of a muffin comes out clean.
4. Carefully remove the baked muffins from the oven. Let the muffins stay in the muffin tin for 5 minutes before you remove them.
5. After 5 minutes, take the muffins out of the tin and place them on a wire rack to cool completely.