In a small bowl, combine ⅓ cup flour, ¼ cup oats, 3 Tablespoons melted butter, 3 Tablespoons brown sugar, and ½ teaspoon cinnamon. Mix until combined and crumbly, and set aside.
Make the Muffins
Preheat the oven to 375°F (190℃) and line a 12-cup muffin pan with muffin liners. Set aside.
In a small bowl, whisk together the flour, cinnamon, baking powder, nutmeg, baking soda and salt. Set aside.
In a large bowl, use a hand-held mixer on medium speed to cream together the softened butter and sugars until light and fluffy, about 2 minutes. Add the sourdough discard, eggs, vegetable oil, yogurt, and vanilla extract, and mix until combined.
Mix the dry ingredients into the wet ingredients and stir until almost no dry spots remain. Fold in the diced apples.
Use a spoon or cookie scoop to fill 12 lined muffin cups about ⅔ full with batter. If you have extra batter, use it to make an additional 1-2 muffins rather than overfilling the pan. Sprinkle generously with streusel topping and turbinado sugar (optional).
Bake for 25-28 minutes, until lightly golden brown and a toothpick inserted in the center comes out clean. Let cool for 5 minutes in the pan, then serve or transfer to a cooling rack to cool fully.
Video
Notes
To make this recipe with active sourdough starter, replace the sourdough discard with active starter (200g). You do not need to make any other changes to the recipe. A previous version of this recipe did not include the streusel topping. This topping is optional, and you can definitely make these muffins without it! If omitting the streusel, I recommend sprinkling the top of the muffins with turbinado sugar before baking.