Wash the lemons thoroughly then pierce all over with a knife. Place the lemons in a large pot and cover with water.  Bring to a full boil, then reduce the heat to medium/low and cover with a lid. Simmer for 45 minutes (lemons should be very soft and tender). Drain and let cool.

Cut the lemons into quarters, discard the seeds, and puree in a blender (rinds and all). The final weight of lemon puree should be 365g.

Preheat the oven to 350 degrees Fahrenheit and spray an 8" cake pan with cooking spray and line with a parchment paper circle.

In a large bowl, add the lemon puree and egg whites (should measure 2⅓ cups). Pour into a large bowl and whisk together.

Whisk in the erythritol and lemon flavor.

In a small bowl, whisk together the pistachio flour, oat flour and baking powder. Slowly add this to the wet ingredients while whisking (batter should be thick, dense, and heavy).

Scoop batter into the prepared pan and spread it out.  Bake for ~55 minutes, or until surface of cake is firm and springs back when tapped. Flip cake onto a wire cooling rack and let cool completely.  Slice and serve!