To prepare the zucchini for scarpaccia, use a mandolin to get evenly-sized very-thin zucchini slices. Repeat with the onion. This can be done by carefully using a sharp knife.

Once both the zucchini and onion are sliced, place into a large mixing bowl and let stand for at least 45 minutes. The longer you let sit, the more excess water you'll have. This is very important so the tart isn't soggy! If time permits, let sit for 2 hours up to overnight in the fridge.

Transfer the zucchini and onion to a cheesecloth or thin, clean kitchen towel. Squeeze any extra liquid into the large bowl and set the vegetables aside. Preheat the oven to 400F.

To the mixing bowl, add the flour, cornmeal, nutritional yeast, rosemary, salt, and water. Add additional water one tablespoon at a time until a batter forms.

Add the vegetables to the batter and stir to coat. Transfer to a 13x18 baking sheet lined with parchment paper. Spread the mixture out into an even, thin layer. Give the top a quick spray of cooking oil.

Bake at for 35-40 minutes, until the edges are slightly golden brown and crispy. Remove from oven and let cool for 10 minutes before slicing and serving.