Preheat oven to 375°F (190°C). Line a baking tray with parchment paper and lightly brush with olive oil.
Thinly slice zucchini (and onion if using). Sprinkle lightly with salt and let sit for 20 minutes. Pat dry with paper towels to remove excess moisture.
In a large bowl, whisk together flour, water, olive oil, salt, and black pepper until smooth.
Fold zucchini (and onion) into the batter. The mixture should lightly coat the vegetables, not form a thick dough.
Spread the mixture evenly and thinly onto the prepared baking tray.
Drizzle a small amount of olive oil on top. Sprinkle herbs and parmesan if using.
Bake for 30–35 minutes, or until golden brown and slightly crisp around the edges.
Remove from oven and let rest for 10 minutes before slicing.
Serve warm or at room temperature.
