Preheat oven to 325F (163C).
Line a 7" square (or similarly sized) pan with parchment paper (if not using a silicone pan).
Mix applesauce and coconut oil until well combined.
Mix in remaining ingredients for the base and mix until smooth.
Spoon into parchment-lined pan, and spread it evenly.
Bake for about 14 minutes, until almost baked and firm.
Meanwhile, cut the ends off the whole lemon and throw them away.
Cut the rest of the lemon into chunks and take out any seeds.
In a high-powered blender or food processor, blend the lemon chunks with the lemon juice until smooth (or not, lemon bits are ok!)
Add remaining filling ingredients and blend to combine.
Remove the pan from the oven (when base is ready) and pour filling mixture on top.
Lower the oven temperature to 300F (150C), and bake for about 8 minutes. Check to see if the filling has stopped jiggling. If not, check every 1-2 minutes.
Let cool at room temperature, and then let cool completely in the refrigerator.
Top with some additional confectioner's style erythritol, and enjoy!