Preheat your oven to 180°C (350°F). In a medium bowl, combine the almond meal, coconut flour, erythritol, salt, lemon zest, and ground cinnamon. Stir in the melted and cooled butter until the mixture resembles wet sand with no dry pockets. Press this mixture firmly and evenly into the bottom of a 9-inch springform pan or similar baking dish, creating an even layer. Bake for 5-8 minutes until light golden and set, then remove from the oven and let cool slightly while you prepare the filling.
In a bowl, toss the sliced rhubarb with the powdered erythritol and lemon juice until the rhubarb is evenly coated. This maceration helps draw out the rhubarb's natural juices and develop flavor. Spread the rhubarb mixture evenly over the baked crust from Step 1, then return to the oven and bake for 15 minutes until the rhubarb softens and releases its juices. Remove from the oven and allow to cool to room temperature—I find this prevents the cheesecake topping from becoming too warm and developing an unwanted texture.
While the rhubarb layer cools, beat the chilled heavy whipping cream with an electric mixer on medium-high speed until soft peaks form, about 2-3 minutes. Add the vanilla essence and sifted powdered erythritol, then continue beating until stiff peaks form. In a separate bowl, beat the room-temperature cream cheese until smooth and creamy, about 1-2 minutes. Gently fold the whipped cream into the cream cheese in two additions, being careful not to deflate the mixture—I like to use a rubber spatula and fold from the bottom of the bowl upward to maintain the airy texture that makes this topping light and cloud-like.
Once the rhubarb layer has cooled to room temperature, spread the cheesecake topping from Step 3 evenly over it, working gently to avoid disturbing the rhubarb beneath. Refrigerate the assembled cheesecake for at least 4 hours, or preferably overnight, until the filling is completely set and chilled. This resting time allows the flavors to meld and gives the cheesecake the proper texture for serving.
