In a small saucepan, melt 1 tbsp butter over medium heat. Add the sliced apples, 2 tbsp sugar, cinnamon, and nutmeg (if using). Cook, stirring occasionally, for 5-7 minutes until apples are slightly tender but still hold their shape. Add 1 tbsp water or apple juice and simmer for another 1-2 minutes until slightly reduced. Remove from heat and set aside.

Preheat your oven to 425°F (220°C). Place the 10-inch cast iron skillet in the oven while it preheats for at least 10-15 minutes. This is crucial for the pancake to puff properly.

In a large mixing bowl, whisk together the eggs until frothy. Gradually whisk in the flour until just combined and smooth, ensuring no lumps. Then, whisk in the milk, 1 tbsp sugar, vanilla extract, and salt until the batter is smooth. Do not overmix.

Carefully remove the hot cast iron skillet from the oven. Add 3 tbsp melted butter to the hot skillet, swirling to coat the bottom and sides evenly.

Immediately pour the prepared pancake batter into the hot, buttered skillet. Return the skillet to the preheated oven.

Bake for 20-25 minutes, or until the pancake is dramatically puffed, golden brown around the edges, and set in the center. The center might still be slightly custardy. Do not open the oven door during the first 15 minutes of baking to ensure maximum puff.

Once baked, carefully remove the German pancake from the oven. It will start to deflate shortly after coming out. Spoon the warm apple cinnamon topping over the center of the pancake. Dust generously with powdered sugar and serve immediately with maple syrup on the side, if desired.