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.

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.

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 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 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.