Preheat your oven to 350°F (175°C) and place the rack in the center position. Grease a 9x5-inch loaf pan thoroughly, then line with parchment paper, leaving overhang on the long sides for easy removal.
In a large bowl, whisk together the flour, sugar, baking powder, and salt until evenly combined.
In a separate medium bowl, whisk the eggs until slightly frothy (about 30 seconds), then add the milk, melted butter, vanilla extract, and lemon zest if using. Whisk until the mixture looks uniform and slightly thickened.
Pour the wet ingredients into the dry ingredients and fold gently with a spatula until just combined. Stop when you no longer see dry flour—about 12-15 folds.
In a small bowl, combine the apple butter, cinnamon, nutmeg, and cloves, stirring until well incorporated.
Pour half of the bread batter into the prepared loaf pan, spreading it evenly with a spatula. Spoon half of the apple butter mixture in dollops over the batter, then use a knife to create light swirls. Don't overmix—the goal is visible ribbons of apple butter.
Repeat with remaining batter and apple butter mixture.
If using the optional topping, combine brown sugar, cinnamon, and chopped nuts in a small bowl, then sprinkle evenly over the top of the batter.
Bake for 55-60 minutes, or until a toothpick inserted in the center comes out clean with just a few moist crumbs. If the top begins browning too quickly, loosely cover with aluminum foil after 35-40 minutes of baking.
Let the bread cool in the pan for 15 minutes, then use the parchment paper overhang to lift the loaf onto a wire rack to cool completely before slicing.