Preheat the oven to 350°F (180°C). Grease a 12-cup muffin tin with butter.
2. Step
Make the muffin batter: In a large bowl, combine the flour, apple, brown sugar, cinnamon, salt, baking soda, vanilla, egg, applesauce, plain yogurt, and apple juice. Whisk until well combined.
3. Step
Make the topping: In a medium bowl, combine the flour, brown sugar, butter, oats, and cinnamon. Mix with a fork until crumbly.
4. Step
Divide the muffin batter between the cups of the muffin tin. Top each muffin with 1 tablespoon of the crumble topping.
5. Step
Bake for 30 minutes, or until a toothpick comes out clean when inserted in the center of a muffin.
6. Step
Make the glaze: In a medium bowl, combine the powdered sugar, lemon zest, and lemon juice. Whisk until smooth.
