In a small bowl, combine the flour, oats, light brown sugar, cinnamon, and a pinch of salt.

Pour in the melted butter and stir until the mixture resembles wet sand.

Set aside while you prepare the muffin batter.

In a large mixing bowl, whisk together the flour, baking powder, cinnamon, nutmeg, salt, and baking soda. This is your dry mixture—set it aside.

In a separate large bowl, cream the softened butter and brown sugar together with a hand mixer until light and fluffy.

Add the sourdough discard and mix until combined.

Stir in the sour cream and vanilla extract.

Beat in the eggs, one at a time, mixing well after each addition.

Add half of the dry ingredients to the wet ingredients and mix until just combined.

Add the remaining dry ingredients along with the diced apples. Stir gently until everything is well combined and the apples are coated with batter. A Danish dough whisk, spatula, or mixing spoon works well for this step.

Use a large cookie scoop to portion the muffin batter evenly into the prepared muffin cups.

Sprinkle the crumble mixture generously over each muffin.

Bake at 425°F for 5 minutes to create a high rise.

Without opening the oven door, reduce the heat to 350°F and continue baking for 15–18 minutes, or until a toothpick inserted into the center comes out clean.

Let the muffins cool in the pan for about 5 minutes, then transfer them to a wire rack to cool completely.