Grease 8 1/2- by 4 1/2-inch loaf pan and line with a 7- by 13-inch sheet of parchment paper so that parchment hangs over the long sides of the pan forming a sling.
Adjust oven rack to lower-middle position and preheat to 350°F (177°C).
Place grated zucchini in a colander and press against the sides to extract liquid, or wrap in a clean kitchen towel or cheesecloth and squeeze to remove excess liquid. You should end up with about 6 1/2 ounces (1 1/2 cups; 185g) drained zucchini. Set aside.
In a medium bowl, whisk together all-purpose flour, whole wheat flour, salt, baking soda, cinnamon, baking powder, coriander, and nutmeg until well combined.
In a large bowl, whisk together eggs, granulated sugar, brown sugar, oil, butter, and vanilla until thoroughly combined and sugars are dissolved.
Add flour mixture to egg mixture. Using a flexible spatula, mix just until just incorporated, and no dry flour remains. Fold in zucchini and walnuts (or pecans, dried fruit, or chocolate chips), if using.
Scrape batter into prepared pan. Bake until loaf is domed and golden brown and a cake tester inserted in the center comes out with few moist crumbs attached, or to an internal temperature of around 200°F (93°C), about 1 hour.
Transfer pan to a wire rack to cool for 30 minutes. Run a butter knife along edges to loosen, then lift loaf using overhanging parchment and transfer to a cutting board. Slice with a serrated knife and serve.
Notes
To make muffins, divide batter evenly among 12 lightly greased nonstick muffin cups, adding about 2 ounces (1/4 cup; 60g) to each cup. Bake until tops are domed and golden brown, about 25 minutes.
To grate zucchini, you can use the large holes of a box grater or shred it in your food processor using the grating disc.
Feel free to swap the toasted walnuts with an equal volume of toasted pecans, dried fruit, or chocolate chips.
