Prepare cranberry sauce: In a small saucepan, combine cranberries, erythritol, and water. Simmer for 5–7 minutes until berries burst and the mixture thickens. Let it cool completely.
Make the crust: Preheat oven to 350 °F. Mix almond flour, melted butter, sweetener, vanilla, and salt until crumbly. Press firmly into an 8×8-inch pan lined with parchment paper. Bake for 10 minutes, then cool slightly.
Make the filling: In a bowl, beat cream cheese with erythritol until smooth. Add egg, sour cream, vanilla, and lemon zest. Mix until creamy.
Assemble: Pour cream cheese mixture over the crust and spread evenly. Drop spoonfuls of cranberry sauce on top and swirl with a knife to create a marbled effect.
Bake: Return to the oven and bake for 20–25 minutes, until the center is mostly set but slightly jiggly.
Cool: Let the bars cool completely, then refrigerate for at least 2 hours before slicing.
Serve: Cut into squares, garnish with extra cranberry sauce or powdered sweetener if desired.