Prep the pan and oven. Heat the oven to 325°F (165°C). Line an 8x8-inch pan with parchment so it overhangs on two sides for easy lifting.

Make the crust. In a bowl, mix almond flour, granulated sweetener, and salt. Stir in melted butter, vanilla, and optional lemon zest until the mixture looks like damp sand. Press evenly into the lined pan. Bake for 10–12 minutes until set and lightly golden at the edges. Set aside to cool slightly.

Cook the blueberry swirl. In a small saucepan, add blueberries, water, sweetener, and lemon juice. Simmer over medium heat, mashing some berries as they soften. Cook 4–6 minutes until thick and jammy. Remove from heat and cool to room temp. If you prefer a smoother swirl, blend briefly and cool.

Beat the cheesecake base. In a large bowl, beat softened cream cheese on medium speed until smooth and fluffy, about 1–2 minutes. Add powdered sweetener and a pinch of salt, and beat until creamy. Mix in eggs one at a time, just until combined. Add sour cream, lemon juice, lemon zest, and vanilla. Beat on low until smooth, avoiding too much air.

Assemble. Pour the cheesecake mixture over the par-baked crust and smooth the top. Spoon small dollops of the cooled blueberry mixture across the surface. Use a skewer or butter knife to gently swirl. Don’t overmix; aim for clean ribbons of color.

Bake. Bake at 325°F for 25–32 minutes. The edges should be set and the center should still have a slight wobble. Avoid browning on top.

Cool and chill. Cool in the pan on a rack for 1 hour. Then refrigerate, uncovered, for at least 3 hours, preferably overnight, until fully set.

Slice and serve. Lift the slab out with the parchment. Use a sharp knife wiped clean between cuts to slice into 12–16 bars. Serve chilled with extra lemon zest or a few fresh blueberries if desired.