Preheat oven to 350°F (175°C). Line a 9x5 loaf pan with parchment and lightly grease.
In a medium bowl whisk almond flour, coconut flour, sweetener, baking powder, baking soda, cinnamon, nutmeg, xanthan gum (if using), and salt until uniform.
In a separate bowl beat eggs, then whisk in melted butter, sour cream, vanilla, lemon zest, and grated zucchini (squeezed to remove excess moisture).
Fold wet ingredients into dry just until combined. Stir in optional nuts or chocolate chips. Transfer to loaf pan and bake 45–55 minutes, until a toothpick comes out mostly clean.
Cool in pan 10–15 minutes, then transfer to a rack to cool completely before slicing so the crumb firms up.
Notes
If almond flour is coarse, pulse it briefly in a processor for a finer texture.
For nut‑free option, try a nut‑free keto blend and adjust coconut flour slightly; results may vary.
Freezes well: slice and freeze between parchment sheets for quick breakfasts.