Grate the zucchini. To use it in the dough they must be as dry as possible, so remove as much moisture as possible out of them by pressing it against the strainer. But gently, you don’t want to mash them. Or you can gather the mass of grated zucchini in a cheesecloth or clean kitchen towel, and squeeze the liquid, again – gently, out. When you’re done, put the dried zucchinis back in a strainer, and over a bowl until you’ll need them.
For the starter, mix the yeast, the sugar (or the honey), the sourdough extract, the water and 100 g flour. Leave for 15 to 30 minutes in a warm place. It should rise a bit and get bubbly. Add another 400 g flour and a generous pinch of salt and knead it until smooth, then mix with the grated zucchinis. The dough will feel pretty squishy, which is okay, but it shouldn’t be all too muddy. You can add some flour, but you shouldn’t use more than a total of max 600 g flour.
Bake at 175°C – 180 °C for one and half hour, or even two. When it’s ready, the crust should feel stone-hard, and it sounds hollow when you knock on it. This a bread with a very crisp crust, and an earthy, spicy taste. It needs time, good nerves and implies a sustained dialog with your oven. But the trick really lies in how much moisture you have squeezed out from the zucchinis. Ambitious testing and practicing help too.
Enjoy and lots of luck!