Preheat oven to 350°F (175°C). Grease a 9-inch pie pan lightly. Zest lemons first, then juice to get ½ cup.
In a bowl, mix almond flour + 2 tbsp powdered erythritol + pinch salt. Add melted butter and stir until mixture holds when pressed. Press into the pie pan, covering base and sides. Bake 8–10 minutes until lightly golden. Cool slightly.
In a large bowl, whisk 4 eggs with ½ cup powdered erythritol for 1–2 minutes until pale. Slowly whisk in lemon juice, then add lemon zest. Whisk in heavy cream until smooth. Taste and adjust sweetener.
Pour filling into the pre-baked crust; smooth top. Bake 20–25 minutes until edges are set and center jiggles slightly. Avoid overbaking.
Cool on a rack 30 minutes, then refrigerate at least 2 hours (overnight best). For clean slices, run a warm knife between cuts.