Prep the pan and oven: Heat oven to 325°F (163°C). Line an 8x8-inch baking pan with parchment, leaving overhang on two sides for easy lifting. Lightly grease the sides.

Make the crust: In a bowl, mix almond flour, powdered sweetener, and salt. Stir in melted butter, vanilla, and optional lemon zest. Press firmly and evenly into the prepared pan. Bake for 10–12 minutes until lightly golden at the edges. Let cool slightly.

Soften the berries: In a small saucepan, add blueberries, sweetener (if using), and lemon juice. Cook over medium heat for 3–4 minutes, just until some berries burst and release juices. Lightly mash to thicken. Stir in optional zest. Cool to lukewarm.

Beat the cheesecake base: Using a hand mixer, beat softened cream cheese and powdered sweetener on medium speed until smooth and fluffy, about 1–2 minutes. Scrape the bowl.

Add flavor and eggs: Mix in sour cream, lemon juice, lemon zest, vanilla, and a pinch of salt until silky. Add eggs one at a time on low speed, mixing just until combined. Do not overmix to avoid introducing excess air.

Assemble: Pour the cheesecake batter over the par-baked crust and smooth the top. Spoon small dollops of the blueberry mixture across the surface. Use a skewer or butter knife to gently swirl, avoiding deep cuts into the crust.

Bake: Bake at 325°F (163°C) for 25–32 minutes, until the edges are set and the center has a slight wobble. Do not overbake—it will continue to set as it cools.

Cool gradually: Turn off the oven, crack the door, and let the bars rest for 10 minutes. Move the pan to a cooling rack and cool to room temperature.

Chill: Cover and refrigerate for at least 4 hours, preferably overnight, until fully set. This step ensures clean slices and the best texture.

Slice and serve: Lift out using the parchment overhang. Wipe a sharp knife between cuts for neat squares. Finish with a sprinkle of lemon zest or a few fresh blueberries if you like.