Preheat oven to 350°F (175°C). Grease a 9x5 inch loaf pan and line it with a parchment paper sling along the bottom and long sides, leaving overhang for easy removal.
In a small bowl, stir together 3 tablespoons of granulated sugar and 1½ teaspoons of ground cinnamon until fully combined. Set aside — you'll use half for the swirl and half for the topping.
Peel, core, and dice 2 medium apples into ½-inch cubes — you should have about 1½ cups. Granny Smith, Honeycrisp, or Fuji apples work best as they hold their shape during baking.
In a large bowl, whisk together the flour, baking soda, baking powder, salt, 1½ teaspoons cinnamon, and nutmeg until evenly combined. Set aside.
In a separate large bowl, whisk together both sugars, eggs, oil, sour cream, and vanilla extract until smooth and slightly pale, about 1 minute.
Pour the dry ingredients into the wet ingredients and fold gently with a rubber spatula just until no streaks of flour remain — about 15 to 20 strokes. Do not over-mix.
Add the diced apples to the batter and fold in gently with 5 to 8 strokes until evenly distributed throughout the batter.
Pour half the batter into the prepared pan, sprinkle with half the cinnamon sugar mixture, then add the remaining batter and top with the remaining cinnamon sugar. Optionally drizzle with melted butter and swirl gently with a knife 2 to 3 times.
Bake on the center rack for 55 to 65 minutes, until a toothpick inserted into the center comes out clean or with a few moist crumbs. If the top browns too quickly, tent loosely with foil after 40 minutes.
Cool the bread in the pan on a wire rack for 15 to 20 minutes, then lift out using the parchment sling and cool for at least 30 to 45 more minutes before slicing with a serrated knife.