Preheat your oven to 350 degrees F and grease a 9-inch springform pan with butter, sprinkling with a little maple sugar.
Heat a 12-inch cast-iron skillet over medium heat and melt 2 tablespoons of butter.
Add the diced apples and 1 tablespoon of maple sugar to the skillet.
Saute the mixture for 5 minutes until the apples are caramelized but still firm, then remove from heat to cool.
In a large bowl, combine the cornmeal, remaining flour, oats, remaining maple sugar, cinnamon, and salt.
Cut the cold, diced butter into the dry mixture with a fork until coarse crumbs form.
Stir in the creme fraiche and maple syrup to create the cake base.
Spread the base into the prepared pan and top with the caramelized apples.
Sprinkle the remaining crumble mixture over the top.
Bake for 45-50 minutes until golden brown and set.