Prep Oven & Pan: Preheat oven to 350°F (175°C). Grease an 8.5x4.5-inch (or 9x5-inch) loaf pan and line with parchment paper, leaving an overhang on the sides for easy removal.

Prep Cinnamon Swirl & Apples: In a small bowl, whisk together the dark brown sugar and 1.5 teaspoons ground cinnamon. In a separate bowl, toss the finely diced apples with 1 tablespoon flour, lemon juice, and 0.5 teaspoon cinnamon so every piece is lightly coated.

Whisk Dry Ingredients: In a medium bowl, whisk together the 2 cups flour, baking powder, baking soda, sea salt, and ground nutmeg until evenly combined.

Mix Wet Ingredients: In a large mixing bowl, whisk together melted butter and granulated sugar until well blended. Beat in eggs one at a time, followed by pure vanilla extract, Greek yogurt, and whole milk until smooth and creamy.

Combine Batter: Pour the dry ingredients into the wet mixture. Fold gently with a rubber spatula just until combined and no dry streaks of flour remain (do not overmix, which creates a tough loaf). Gently fold in half of the prepared diced apples.

Layer & Swirl: Spread half of the batter evenly into the bottom of the prepared loaf pan. Scatter half of the remaining diced apples, then sprinkle half of the cinnamon brown sugar mixture over the top. Pour the remaining batter over, smoothing the top. Scatter the final diced apples and remaining cinnamon sugar on top. Gently drag a butter knife or skewer through the batter in an S-pattern 2 to 3 times to create gorgeous marble swirls.

Bake to Golden Perfection: Bake at 350°F (175°C) for 55 to 65 minutes. A wooden toothpick inserted into the center should come out clean or with a few moist crumbs (if the top browns too quickly after 40 minutes, tent loosely with aluminum foil). Let cool in the pan on a wire rack for 15 minutes, then lift out with parchment and cool completely before slicing.