Preparation

• Salt and drain the zucchini and onion: Place the zucchini and onion in a large colander, sprinkle with 1 teaspoon of salt, and toss to combine. Place the colander over a large bowl to collect any liquid (you'll use it later when you make the batter). Weigh down the zucchini mixture with a plate or bowl that fits inside the colander and something heavy, like a couple of cans or jars. Set aside for at least 1 hour and up to 3 hours.
• Preheat the oven to 400°F. When you're ready to cook, preheat the oven. Line a rimmed quarter sheet pan (9x13-inch) with parchment paper and generously drizzle it with olive oil.
• Dry the zucchini: Remove the weights and plate from the zucchini and squeeze the excess water out, pressing the zucchini into the colander over the bowl below. Pour the zucchini water into a measuring cup. Measure out ½ cup, discarding any extra or adding water if you don’t have enough, and add back to the bowl.
• Make the batter: Add the cornmeal, rice flour, and the remaining ½ teaspoon of salt to the bowl with the ½ cup of zucchini water and whisk everything together to form a crumbly batter. Transfer the squeezed zucchini and onions to the batter and stir with a rubber spatula until the zucchini is well-coated.
• Transfer to the pan: Transfer the mixture to the well-oiled sheet pan and press everything down with your hands until evenly spread out. Drizzle generously with olive oil, and sprinkle with a final dusting of cornmeal.
• Bake: Bake until golden brown and crispy, rotating the pan halfway through, 30 to 40 minutes. Set aside to cool for 10 or so minutes, then carefully transfer the tart to a cutting board, using the parchment as handles, and cut into desired serving sizes. Serve immediately.
• Store leftovers in the fridge in a tightly sealed container in even layers with parchment in between. It’ll keep for up to 4 days.