Preheat the oven to 400 °F (200 °C). Grease 5 ramekins with a tablespoon of butter and then coat with granulated sugar with a tablespoon of granulated sugar (use more if needed).
Heat a large skillet. Melt the butter then add the apple and cook until soft. Add the 2 tablespoon maple syrup and toss delicately to coat. Remove from heat and set aside to cool completely.
Meanwhile, whisk together the milk, eggs, 4 tablespoon granulated sugar, 2 tablespoon maple syrup, and the flour. Pour through a fine mesh sieve to work out any lumps, then whisk in the freshly grated nutmeg.
Divide the cooked diced apples between the 5 ramekins, then pour the batter over top. Top each with a slice of cooked apple.
Bake for about 30 minutes, until they are puffed and set.
Let cool slightly before serving. You can brush the tops with a little extra maple syrup and sprinkle with a little more nutmeg before serving.