1. Make the streusel topping:
In a small bowl, combine flour, granulated sugar, brown sugar, and cinnamon. Cut in the cold butter using a pastry cutter or fork until the mixture forms coarse crumbs. Set aside.

2. Prepare the loaf batter:
In a medium bowl, whisk together the flour, baking powder, baking soda, salt, cinnamon, and nutmeg.

In a large bowl, beat the softened butter with granulated and brown sugars until light and fluffy, about 2–3 minutes. Add eggs one at a time, beating well after each addition, then mix in the vanilla extract.

Gradually add the dry flour mixture to the butter mixture, alternating with the sour cream or Greek yogurt, beginning and ending with the flour mixture. Mix until just combined.

Fold in the diced apples until evenly distributed.

3. Assemble the loaf:
Pour half of the batter into a prepared loaf pan. Sprinkle half of the streusel topping over the batter. Repeat with the remaining batter and streusel topping.

4. Bake and enjoy:
Bake according to your preferred temperature and time until a toothpick inserted in the center comes out clean. Let cool slightly before slicing.