Melt 3 Tbsp of butter in a 10" cast iron skillet over medium heat. (The skillet must be ovenproof.)
Peel and quarter the apple. Slice quarters lengthwise into thin strips. Add to the skillet. Sprinkle a tiny bit of salt over the fruit. Fry the apples for a few minutes over medium heat, stirring occasionally so they don't burn. They will be slightly soft about the time the batter is ready.
While the apples are cooking, start the batter. In a medium bowl put milk, cream, eggs, flour, sugar and salt. Whisk to combine.
Melt 2 Tbsp of butter and whisk it into the batter. The batter should be thin and runny with no lumps. Set aside.
Add cinnamon and brown sugar to the apples. Stir to combine making sure to leave the apples in an even layer in the bottom of the skillet.
Pour batter evenly over the apples. Put the skillet in the oven and bake for 10-11 minutes until top is cooked and slightly golden. Flip unto a plate and serve immediately. This is good by itself or with butter and maple syrup.