Prepare the Muffin Pan: Preheat the oven to 400°F. Spray a 12-count muffin tin with cooking spray or line it with paper liners, then set aside.
Combine Dry Ingredients: In a large bowl, combine the all-purpose flour, quick-cooking oatmeal, light brown sugar, ground cinnamon, baking powder, baking soda, and salt.
Mix Wet Ingredients: In a separate bowl, whisk together the eggs, milk or apple cider, unsweetened applesauce, vegetable oil, and pure vanilla extract until combined.
Combine the Batter: Make a well in the center of the dry ingredients. Pour in the wet mixture and stir just until the ingredients are combined, being careful not to overmix.
Add the Apples: Fold the finely chopped apple into the batter until it is evenly distributed.
Fill the Muffin Cups: Divide the batter evenly among the 12 prepared muffin cups.
Prepare the Streusel: In a small bowl, combine the quick-cooking oatmeal, all-purpose flour, brown sugar, melted butter, and ground cinnamon.
Top the Muffins: Sprinkle the prepared streusel mixture evenly over each muffin.
Bake the Muffins: Bake at 400°F for 15 to 20 minutes, or until a toothpick inserted into the center of the muffins comes out clean.
Check for Doneness: Test a muffin at the end of the baking time with a toothpick. The muffins are ready when the toothpick comes out clean.
