Preheat the oven to 325°F and line an 8x8-inch baking pan with parchment paper, leaving overhang on two sides.
Whisk together the almond flour, brown sweetener, baking powder, cinnamon, nutmeg, ginger and salt. Stir in the eggs, melted butter and vanilla until smooth.
Fold the grated carrots and walnuts into the batter. Spread about two thirds of it evenly in the pan.
Beat the cream cheese and powdered erythritol until completely smooth. Add the egg, sour cream, vanilla and lemon juice and mix on low just until combined.
Pour the cheesecake mixture over the carrot cake layer. Drop spoonfuls of the remaining carrot cake batter on top and swirl lightly with a skewer.
Bake for 35 to 40 minutes, until the edges are set and the center has only a slight jiggle.
Cool to room temperature, then chill for at least 3 hours. Lift out and slice into 16 bars.