Preheat oven to 350°F (175°C). Grease the pan and line with parchment paper.
Toss diced apples with lemon juice; set aside.
Whisk flour, baking powder, cinnamon, nutmeg, and salt in a bowl.
Cream butter + brown sugar until fluffy. Add eggs one at a time, then vanilla.
Alternate adding dry ingredients and milk to the butter mixture. Fold in apples.
Pour batter into the pan. Sprinkle cinnamon-sugar topping (if using).
Bake 45–50 minutes or until a toothpick comes out clean.
Let cool in the pan for 15 minutes, then transfer to a wire rack. Serve warm or at room temperature.