Preheat oven to 400℉ (204℃). Spray a 12 count muffin tin with cooking spray or line with paper liners and set aside.
In a large bowl, combine the flour, oats, brown sugar, cinnamon, baking powder, baking soda, and salt.
1 1/2 cups (212g) all-purpose flour, 1 cup (233g) quick cooking oatmeal, 3/4 cup (150g) light brown sugar, 1 teaspoon ground cinnamon, 1 1/2 teaspoons baking powder, 1/2 teaspoon baking soda, 1/2 teaspoon salt
In a separate bowl, whisk together the eggs, milk, applesauce, oil, and vanilla.
2 large eggs, 1/2 cup (118ml) milk, 1/2 cup unsweetened applesauce, 1/4 cup vegetable oil, 1 teaspoon pure vanilla extract
Make a well in the center of the dry ingredients and add the wet ingredients, stirring just until combined. Fold in the chopped apple, then divide evenly between the 12 muffin cups.
1 cup finely chopped apple
Combine all of the streusel ingredients together in a small bowl, then sprinkle over each muffin. Bake the muffins for 15 to 20 minutes until a toothpick inserted into the center of the muffins comes out clean.
1/4 cup (58g) quick cooking oatmeal, 2 Tablespoons all-purpose flour, 2 Tablespoons brown sugar, 2 Tablespoons salted butter, 1/4 teaspoon ground cinnamon