Preheat your oven to 350°F (175°C) and position the rack in the middle. Line a 9x5 inch loaf pan with parchment paper, leaving some overhang on the sides for easy removal.
In a medium bowl, whisk together flour, baking soda, salt, cinnamon, nutmeg, and cloves until evenly distributed.
In a large bowl, beat eggs with both sugars until light and fluffy, about 2 minutes on medium-high speed. Add oil, milk, and vanilla extract, then mix until smooth and well combined.
Gradually fold the dry ingredients into the wet mixture, using gentle strokes just until combined. Avoid over-mixing.
Fold in the diced apples. Pro tip: tossing your apple pieces in a tablespoon of flour before adding them prevents them from sinking to the bottom during baking.
In a separate bowl, beat mascarpone cheese with egg, sugar, vanilla extract, and lemon zest until smooth and creamy. Make sure your mascarpone is truly at room temperature to avoid lumps.
Pour half of the apple batter into your prepared loaf pan. Spoon half of the mascarpone mixture on top in dollops. Add remaining apple batter, then top with the rest of the mascarpone mixture. Use a butter knife or skewer to create swirls by gently dragging it through the batters in a figure-eight pattern.
If using streusel topping, combine flour, brown sugar, and cinnamon in a small bowl. Cut in cold butter using a pastry cutter or your fingertips until mixture resembles coarse crumbs. Sprinkle evenly over the top of the batter.
Bake for 55-65 minutes, or until a toothpick inserted into the center comes out with just a few moist crumbs. If the top browns too quickly, tent with aluminum foil after about 35 minutes.
Allow bread to cool in the pan for 15 minutes before using the parchment paper handles to lift it out onto a wire rack. Cool completely (at least 30 minutes) before slicing to allow the mascarpone swirl to set properly.