Preheat oven to 400º. Grease bottoms of muffin tin.
In a mixing bowl, beat egg and stir in milk, apple and oil. Stir in the flour, cinnamon, brown sugar, baking powder and salt all at once and stir until just moistened. The batter will be slightly lumpy. Fill 12 muffin cups about 3/4 full.
In a small bowl, mix together the topping ingredients: 1/4 cup brown sugar, 1/2 tsp. cinnamon and 1/4 cup chopped nuts.
Sprinkle evenly over tops of uncooked muffins. Bake until golden brown, about 16-20 minutes (keep checking to see if they're finished cooking!) Remove pan from oven and immediately remove from muffin tin and cool on a rack. Enjoy!