Preheat the oven to 325F and line a 9x5 inch loaf pan with parchment paper. Whisk together the almond flour, coconut flour, granulated erythritol, poppy seeds, baking powder, and salt in a large bowl.
In a separate bowl, whisk the eggs, melted butter, sour cream, lemon juice, lemon zest, and vanilla extract until smooth and combined.
Pour the wet ingredients into the dry ingredients and stir until a smooth, thick batter forms. Let the batter rest for 3 minutes so the coconut flour absorbs the moisture.
Pour the batter into the prepared pan and smooth the top with a spatula. Tap the pan on the counter twice to release air bubbles. Bake for 50 to 55 minutes until the top is golden and a toothpick inserted into the center comes out clean. If the top browns too quickly, tent loosely with foil after 35 minutes.
Let the loaf cool in the pan for 15 minutes, then lift out using the parchment and place on a wire rack to cool completely.
Whisk together the powdered erythritol, lemon juice, melted butter, and lemon zest for the glaze. Drizzle generously over the cooled loaf, letting it drip down the sides. Let the glaze set for 10 minutes before slicing.
