Preheat your oven to 350°F (175°C) and line a 9x5 inch loaf pan with parchment paper or lightly grease it.

In a large bowl, whisk together the almond flour, coconut flour, erythritol, baking powder, baking soda, salt, cinnamon, and nutmeg. Ensure all dry ingredients are well combined and free of clumps.

In a separate bowl, beat the eggs until frothy. Mix in the melted butter and vanilla extract until well incorporated.

Add the wet ingredients to the dry ingredients, stirring until just combined. Be careful not to overmix.

Fold in the grated zucchini and optional walnuts until evenly distributed throughout the batter.

Pour the batter into the prepared loaf pan, smoothing the top with a spatula.

Bake in the preheated oven for approximately 60 minutes, or until a toothpick inserted into the center comes out clean.

Allow the bread to cool in the pan for 10 minutes before transferring to a wire rack to cool completely.

Slice the bread once cooled and serve. For best results, store any leftovers in an airtight container in the refrigerator.