Preheat oven to 425 degrees. In a bowl, whisk together eggs, milk, flour, and salt; set aside. In a large ovenproof skillet, melt butter over medium heat. Cook apple slices, stirring occasionally, until soft, 6 to 8 minutes. Add brown sugar and cinnamon and cook until caramelized, 2 to 3 minutes.
Pour in batter and transfer to oven. Bake until cake puffs, about 12 minutes. Sprinkle with raw sugar and return to oven until brown on top, 2 to 3 minutes. Let cool slightly. Serve each slice with a dollop of Greek yogurt, if using.