Prep the pan and oven. Preheat your oven to 350°F (175°C). Lightly grease a 9-inch tart pan with a removable bottom, or line the base with parchment for easy release.

Make the crust. In a bowl, whisk the almond flour, sweetener, and salt. Stir in melted butter and vanilla until the mixture looks like damp sand. If using, mix in the egg white to help bind.

Press and par-bake. Press the crust evenly into the pan, going up the sides. Dock the base with a fork to prevent puffing. Bake for 12–14 minutes, until lightly golden at the edges. Set aside to cool slightly.

Whisk the filling. In a medium saucepan off heat, whisk the eggs, sweetener, lemon juice, zest, cream, vanilla, and salt until smooth. Place over low to medium-low heat.

Cook gently. Stir constantly with a silicone spatula, scraping the sides and bottom. Cook until the mixture thickens to a loose curd consistency and coats the back of a spoon, about 6–10 minutes. Do not let it boil hard.

Add butter for silkiness. Remove from heat and whisk in the butter pieces until fully melted and glossy. For an ultra-smooth finish, strain the filling through a fine mesh sieve into a bowl.

Fill the crust. Pour the warm lemon filling into the par-baked crust and smooth the top. Tap the pan gently on the counter to release any air bubbles.

Bake to set. Return to the oven and bake for 10–12 minutes, just until the edges are set and the center still has a slight wobble. It will firm up as it cools.

Cool and chill. Let the tart cool to room temperature, then refrigerate for at least 3 hours, preferably overnight, until fully set.

Garnish and serve. Dust with powdered keto sweetener, add lemon slices or a few berries, and slice with a warm, dry knife for clean edges.