Cook the apple cider in a pot over medium heat until reduced to half—¾ cup. Set aside.
Preheat the oven to 350°F (180°C). Lightly grease and line a 9×5 loaf pan with parchment paper. Set aside.
In a bowl, whisk together the flour, cinnamon, salt, baking powder, baking soda, and nutmeg. Set aside.
Grate 1-2 red apples into a bowl. Squeeze out the juice, then measure 1 cup of grated apples. Discard the juice or drink it.
In a large mixing bowl, whisk together the oil, brown sugar, eggs, grated apples, and reduced apple cider. Whisk in the dry ingredients until just combined.
Pour the batter into the prepared loaf pan.
In a small bowl, combine the granulated sugar and cinnamon. Sprinkle two tablespoons of the mixture over the top.
Bake for 50-60 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.
Let the pan cool on a wire rack for 15 minutes, then lift the parchment up to remove it. Brush melted butter over the sides of the cake and cover with the remaining cinnamon sugar. Cut into 8-10 slices and enjoy!