This recipe makes two loaves, but can easily be halved. If making the full recipe, the second loaf will freeze for up to 3 months, well-wrapped.
Preheat oven to 350°F. Grease two 9x5 inch loaf pans with butter or olive oil and line with parchment paper.
In a medium bowl, whisk together flour, baking powder, baking soda, salt, and cardamom. Set aside.
In a small skillet over medium heat, melt the butter and continue to cook, stirring occasionally, until a nutty aroma develops and butter becomes brown, 5-7 minutes. Once the butter stops foaming, add to a large heat-proof bowl with the Colavita Essential olive oil, and allow to cool for a few minutes.
While cooling the butter-olive oil mixture, grate the zucchini on the large holes of a box grater or shred in a food processor. Wrap in a clean kitchen towel and squeeze out as much moisture as possible. Discard liquid.
Whisk the eggs, brown sugar, and white sugar into the butter-olive oil mixture until emulsified and smooth, about 2 minutes. Stir in the grated zucchini and lemon zest.
Add the flour mixture to the zucchini mixture in 3 additions, folding gently without overmixing. If using, fold in the walnuts when just a few dry streaks of flour remain until everything is fully incorporated.
Divide the batter evenly among the prepared pans and smooth the tops. Top with demerara sugar, if using. Bake, rotating pans halfway through, until a toothpick inserted into the center comes out clean, 50-60 minutes.
Cool the loaves in the pans for 10 minutes before removing and peeling the parchment paper off. Set on a wire rack to cool completely before slicing.