Preheat oven to 350°F. Line 12 muffin cups with muffin linings. Set aside.
Peel 1 medium apple, then grate it on a large holed grater.
In a large bowl, combine brown sugar, eggs, vegetable oil, and vanilla extract. Fold in flour, baking powder, baking soda, and salt. Fold in shredded apples.
In a separate bowl, combine crumble ingredients- brown sugar, flour, oatmeal, cinnamon, and a pinch of nutmeg. Use a pastry cutter to work in butter until texture resembles a fine sand.
Scoop batter into muffin cups, spreading evenly between 12 muffin cups. Crumble oatmeal topping over batter.
Bake muffins for 20-25 minutes, then allow to cool sightly before serving!