Preheat oven to 320F. Line the bottom and sides of a 7-inch round springform pan with parchment paper. The parchment should come up at least 2 inches above the rim of the pan. Set a large deep baking dish on the lower rack and fill it halfway with hot water to create a water bath.
Melt the cream cheese and butter together in a saucepan over low heat, stirring until completely smooth. Remove from heat and whisk in the heavy whipping cream, lemon juice, and vanilla extract.
Let the mixture cool for 5 minutes, then whisk in the egg yolks one at a time until fully incorporated.
Sift the almond flour and coconut flour into the cream cheese mixture and fold gently until no lumps remain.
In a large clean bowl, beat the egg whites with cream of tartar and salt on medium speed until foamy. Gradually add the powdered erythritol a little at a time, beating on high until the meringue reaches stiff, glossy peaks. This should take about 4 to 5 minutes.
Fold one-third of the meringue into the cream cheese batter to lighten it. Then gently fold in the remaining meringue in two additions using slow, sweeping bottom-to-top motions. Do not rush this step as the air in the meringue is what makes the cheesecake fluffy.
Pour the batter into the prepared pan and gently tap it on the counter twice to release any large air bubbles. Place the pan on the rack above the water bath.
Bake for 55 to 65 minutes until the top is golden and the center has only a very slight jiggle. Turn off the oven, crack the door open slightly, and let the cheesecake cool inside the oven for 30 minutes.
Remove from the oven and let it cool completely at room temperature before peeling off the parchment. The cheesecake will deflate slightly, which is normal. Serve at room temperature or slightly chilled for the fluffiest, most jiggly texture.
