Preheat and Prep: Preheat your oven to 350˚F (175°C). Line a standard 9x5-inch loaf pan with parchment paper, leaving an overhang on the sides for easy removal. Set aside.
Mix the Cinnamon Sugar: In a small bowl, whisk together the light brown sugar, ground cinnamon, and the pinch of nutmeg until well combined. Set aside.
Combine Wet Ingredients: In a large mixing bowl, whisk together the granulated sugar, melted and cooled butter, room-temperature sour cream, eggs, milk, and vanilla extract until completely smooth and creamy.
Combine Dry Ingredients: In a separate medium bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt.
Form the Batter: Gently pour the dry ingredients into the wet ingredients. Use a spatula to fold them together until just combined. Be careful not to overmix!
Add the Apples: Fold all but a small handful of the diced Granny Smith apples into the batter.
Layer and Swirl: Pour half of the batter into the prepared loaf pan and smooth it into an even layer. Sprinkle half of the cinnamon-sugar filling evenly over the batter. Take a flat butter knife and swirl it through the batter in a figure-eight motion.
Top It Off: Gently spread the remaining batter over the top, smoothing it out. Scatter the reserved handful of diced apples over the batter, and sprinkle the remaining cinnamon-sugar mixture on top. Swirl once more with the knife.
Bake: Bake in the preheated oven for 50-55 minutes, or until a toothpick inserted into the center of the loaf comes out clean. (If the top browns too quickly, loosely tent with foil for the last 15 minutes).
Cool: Remove from the oven and allow the bread to cool in the pan for 15-20 minutes before using the parchment paper overhang to lift it onto a wire rack to cool further.
Glaze: While the bread cools, whisk together the sifted icing sugar and 1 teaspoon of hot water. If the glaze is too thick, add the second teaspoon of hot water until it reaches a smooth, drizzly consistency. Drizzle the glaze over the warm loaf. Let the glaze set for 5-10 minutes before slicing and serving. Enjoy!