For the Coffee Cake:
Preheat your oven to 350°F (175°C). Grease a 10-inch cast-iron skillet.
In a medium bowl, whisk together flour, baking powder, baking soda, nutmeg, and salt. Set aside.
In a large bowl, cream together softened butter and granulated sugar until light and fluffy.
Beat in the eggs, one at a time, ensuring each is fully incorporated. Add the vanilla extract and mix well.
Gradually add the dry ingredients to the wet ingredients, alternating with sour cream. Begin and end with the dry ingredients, mixing until just combined.
Gently fold in the diced apples until evenly distributed in the batter.
Spread the batter evenly in the prepared cast-iron skillet.
For the Streusel Topping:
In a small bowl, combine flour, brown sugar, and ground cinnamon.
Add cold, cubed butter and use a pastry cutter or your fingers to incorporate until the mixture resembles coarse crumbs.
Stir in the chopped nuts.
Sprinkle the streusel topping over the coffee cake batter.
Bake for 35-40 minutes or until a toothpick inserted into the center comes out clean. Allow the coffee cake to cool slightly before serving.