Preheat the oven to 425℉. In a medium bowl, toss the apple slices, 2 tablespoons of the sugar, and 1/4 teaspoon cinnamon.
Put the butter in a 9-inch cast-iron skillet. Place the skillet in the oven for 2 minutes, until the butter melts.
Take the skillet out of the oven, swirl the melted butter around the pan, and add the apple mixture.
Return to the oven and bake for 10 minutes, or until the apples begin to soften and the butter is bubbling.
While the apples are cooking, add the eggs, milk, flour, vanilla, salt, and remaining 1 tablespoon of sugar and 1/2 teaspoon of cinnamon to the blender. Blend for 1 minute until frothy.
Pour the batter over the hot apples.
Bake the popover for 25 to 30 minutes, until puffy and golden brown.