Preheat oven to 350°F (175°C). Grease a 9×5-inch loaf pan and line with parchment paper. Set aside.
Peel, core, and dice the apples into ¼-inch pieces. Toss with 1 tsp cinnamon in a small bowl and set aside.
In a large bowl, whisk together the sourdough discard, eggs, oil, brown sugar, and vanilla extract until smooth and fully combined.
Add the flour, baking soda, baking powder, cinnamon, nutmeg, and salt to the wet ingredients. Fold with a rubber spatula until just combined — do not overmix.
Gently fold in the cinnamon-coated apple pieces until evenly distributed throughout the batter.
Pour the batter into the prepared loaf pan and smooth the top. Mix the cinnamon sugar topping and sprinkle evenly over the surface.
Bake for 55–65 minutes, or until a toothpick inserted into the center comes out clean. If the top browns too quickly, tent with foil after 40 minutes.
Cool in the pan for 15 minutes, then transfer to a wire rack for at least 30 minutes before slicing. Enjoy!