Preheat oven to 350°F. Grease a 9x5 inch loaf pan.
In a small bowl, mix together the brown sugar, cinnamon, and chopped pecans for the swirl mixture. Set aside.
In a large bowl, whisk together the flour, baking powder, salt, and sugar.
In a separate bowl, whisk together the milk, egg, applesauce, oil, and vanilla. Pour the wet ingredients into the dry and stir gently until just combined. Fold in the diced apple.
Pour half of the batter into the prepared loaf pan. Sprinkle with two-thirds of the cinnamon sugar pecan mixture.
Pour the remaining batter on top and spread gently with a spatula. Sprinkle with the remaining cinnamon mixture.
Use a knife to draw a figure-8 pattern through the top layer of batter to create the swirl. Make 2-3 passes only.
Bake for 60-70 minutes, covering loosely with foil after 30 minutes to prevent over-browning. The bread is done when a knife inserted in the center comes out with a few moist crumbs.
Cool on a wire rack before slicing.
Notes
Swirl tip: Only make 2-3 passes with the knife. Over-swirling blends the cinnamon sugar into the batter instead of leaving distinct ribbons.
Measure the apple: Use exactly 1 cup diced. More apple can make the center too moist.
Flour swap: Use half whole wheat flour for a nuttier flavor, or a 1-to-1 GF blend for gluten-free.
Storage: Room temperature for 3 days, refrigerator for 1 week, or freeze tightly wrapped for up to 3 months.
Optional crumb topping: Mix 2 tablespoons cold butter, ¼ cup flour, 2 tablespoons brown sugar, and 1 teaspoon cinnamon until crumbly. Sprinkle on top before baking.