Step 1

Preheat oven to 350 degrees. Line a standard 12-cup muffin tin with baking cups. In a medium bowl, whisk together flour, oats, baking powder, salt, and cinnamon. In a separate bowl, whisk together sugar, eggs, butter, and milk. Stir wet ingredients into dry ingredients, then stir in apple.

Step 2

Divide batter evenly between muffin cups, adding a scant 1/4 cup to each. Sprinkle with crumb topping if desired. Bake until tops spring back when lightly touched, about 20 minutes. Cool in pans 5 minutes, then remove to wire rack to cool completely.