Preheat oven to 350°F (175°C). Grease and line a 9x5-inch loaf pan with parchment paper, leaving an overhang for easy removal.

In a medium bowl, combine flour, baking powder, baking soda, salt, 1 tsp cinnamon, and nutmeg. Set aside.

In a small bowl, stir together brown sugar, 1 tbsp cinnamon, and melted butter to make the swirl mix. Set aside.

In a large bowl or stand mixer, beat the softened butter with granulated sugar and 1/3 cup brown sugar until light and fluffy (about 2–3 minutes).

Beat in eggs one at a time, then stir in vanilla and lemon zest. Scrape down the bowl as needed.

Add one-third of the dry mixture, then half the yogurt, then another third of dry, the rest of the yogurt, and finish with remaining dry. Mix until just combined — don’t overmix.

Gently fold the chopped/grated apple into the batter.

Spread half the batter into the prepared pan. Sprinkle half the cinnamon swirl mixture evenly over batter. Spoon remaining batter on top, smoothing gently. Sprinkle remaining swirl mixture.

Using a butter knife, create 6–8 figure-eight swirls through the batter — don’t overdo it or the swirl will blend away.

Bake for 50–60 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs. Tent with foil if the top browns too quickly.

Let the loaf cool in the pan for 15 minutes, then lift out and transfer to a rack. Cool at least 30 minutes before glazing or slicing.

Whisk powdered sugar, milk, vanilla, and a pinch of cinnamon until smooth. Drizzle over cooled loaf and let set before slicing.

Use a serrated knife for clean slices. Serve slightly warm with butter or a smear of cream cheese.