Prep the pan: Heat the oven to 325°F (163°C). Line the bottom of a 9-inch springform pan with parchment paper and lightly grease the sides.

Make the crust: In a bowl, mix almond flour, sweetener, and salt. Stir in melted butter and vanilla until the mixture looks like damp sand. Press firmly and evenly into the pan. Bake for 10 minutes. Cool while you make the filling.

Cook the berry swirl: In a small saucepan, combine berries, water, and sweetener. Simmer over medium heat for 5–7 minutes, stirring and mashing until thickened. Stir in lemon zest if using. Push through a fine mesh sieve to remove seeds. Cool to room temperature.

Beat the cream cheese: In a large bowl, beat cream cheese on medium speed until smooth and fluffy, about 2 minutes. Scrape the bowl.

Add sweetener and flavor: Beat in powdered sweetener, vanilla, lemon juice, and salt until fully combined. Keep scraping the bowl so everything blends evenly.

Mix in sour cream: Beat in sour cream on low until just smooth.

Add eggs gently: Add eggs one at a time on low speed, mixing just until incorporated. Do not overmix; too much air can cause cracks.

Assemble: Pour the filling over the crust. Spoon small dollops of berry puree on top and swirl gently with a skewer or knife to create a marbled look.

Create a water bath (optional but helpful): Wrap the outside of the springform pan with heavy foil. Place it in a larger roasting pan and add hot water to come halfway up the sides. This helps prevent cracks.

Bake: Bake at 325°F for 50–60 minutes, until the edges are set but the center still jiggles slightly. Do not overbake.

Cool slowly: Turn off the oven and crack the door. Let the cheesecake rest inside for 30 minutes. Remove from the water bath, then cool to room temperature.

Chill: Cover and refrigerate for at least 6 hours, preferably overnight. Release from the pan and transfer to a serving plate.

Finish and serve: Add whipped cream, chocolate shavings, or fresh berries. Slice with a hot, dry knife for clean edges.