While the graham cracker crust bakes and cools, make the strawberry cheesecake filling.
Place 3 cups of room-temperature cream cheese in a medium-sized bowl. Beat with a hand mixer or stand mixer with a whisk attachment at medium to medium-high speed. Mix until the cream cheese is smooth and creamy, and no lumps remain.
Add 1 cup of sugar and mix on medium speed until combined, scraping the sides of the bowl with a rubber spatula as needed. Then add 1/2 cup of sour cream, 2 tsp vanilla extract, and 1 tsp lemon zest (optional) and mix at a medium speed until combined.
Next, mix in 3 eggs, one at a time. Mix on a low speed until the eggs are fully incorporated. The mixture should be perfectly smooth at this stage.
Pour about half of the cheesecake batter (550g if you have a scale) into a separate bowl and mix in 1/3 cup of strawberry reduction (made above) on a low speed until combined. If you want the strawberry cheesecake to have a brighter color, you can also mix in a drop of pink or red gel food coloring. I added a tiny bit of red gel food coloring to my batter!
Alternate pouring the regular and strawberry cheesecake batter on the cooled graham cracker crust. Then add spoonfuls of the strawberry reduction on top of the cheesecake batter like in the photos above. Gently drag a knife or offset spatula through the batter to create strawberry swirls. After this, you should have about 1/2 cup of the strawberry reduction left over after topping the cheesecake bars. Save this to spoon on top of the bars once they're baked and cooled.
Bake the cheesecake bars for 50-60 minutes. Do a wiggle test to check if they're done (watch the center, and look for a small wiggle), rather than a toothpick!
Let the bars cool completely in the pan on a metal rack. Once they reach room temperature, refrigerate them for an additional 2 hours or overnight. The strawberry swirls will sink a little bit once the cheesecake is baked and cooled, and I like to spoon the leftover strawberry reduction into any cracks or little crevices to give it a beautiful finished look!
Cut the bars with a sharp, warm knife once they're fully chilled. Clean the knife after each cut to get perfect slices. Leftover bars can be kept in the fridge for up to 5 days in an airtight container.