Preheat the oven to 350F. Grease and flour a 9-inch springform pan.
In a bowl, whisk the flour, ground almonds, baking powder, salt and nutmeg.
In a bowl, with an electric mixer on medium-high speed, beat the eggs, granulated sugar, and vanilla until light and creamy. With the mixer on medium speed, beat in the butter until combined. Fold in the flour mixture by hand. Stir in the milk, then the apple.
Scrape the batter into the pan and bake until golden brown, about 50 minutes. Let cool completely in the pan, then remove the sides of the pan. Dust with a little powdered sugar before serving, if desired, for a sweeter taste.
TIP: Use an apple with soft flesh, like McIntosh or Honeycrisp (I love Honeycrisp), that breaks down quickly in the oven. Grate the apple on the largest holes of a box grater, the size used for grating mozzarella cheese for pizza.