Preparation

• Using a mandolin or sharp knife, slice the zucchini and onion as thinly as possible. Place the vegetables in a medium bowl, then add 1 teaspoon of the salt and use your hands to massage it into the vegetables.
• Transfer the mixture to a fine mesh sieve set over a bowl. Place a small plate on top and add a heavy weight on top, like a large can of tomatoes. Drain for at least 1 hour, or up to 3 hours. (Reserve the drained liquid for step 5.)
• When you’re ready to bake, preheat the oven to 450°F with a rack in the center. Line a baking sheet with parchment, drizzle 1 tablespoon of the olive oil over the parchment, then use a pastry brush or your fingers to spread it evenly. (Be sure to coat the sides of the pan too.)
• In a large bowl, combine the flour, cornmeal, Parmesan, cornstarch, spices, and the remaining ¼ teaspoon salt.
• In a liquid measuring cup, measure the drained liquid from the vegetables, then add enough water to equal 1 ¼ cups plus 1 tablespoon. Add the remaining 1 tablespoon of olive oil and stir to combine.
• Pour the wet ingredients into the dry ingredients and stir until smooth. Add the vegetables and stir until they’re evenly distributed.
• Transfer the mixture to the prepared pan, then use wet hands to distribute it as evenly as possible, all the way to the edges of the pan. The mixture will be very thin.
• Bake the scarpaccia for 28 to 32 minutes, until golden brown.
• Remove it from the oven and carefully invert it onto a metal cooling rack. Peel away the parchment, then return the scarpaccia, still upside-down on the cooling rack, to the oven and bake until deeply colored and crisp, about 10 to 15 minutes.
• Remove from the oven and cool briefly, about 10 minutes. For the neatest presentation, flip the scarpaccia back over so that it's right-side up, then slice and serve.
• Scarpaccia is best eaten warm; store leftovers in an airtight container at room temperature for several days.