Very carefully mandolin the zucchini, red onion, and bell pepper into a large bowl. You can also just use a knife to cut very thinly.
Toss and squeeze sliced vegetables with salt and black pepper.
Transfer sliced vegetables to a cloth or colander and set over the large bowl. The salt will pull out the water from the vegetables. This will take up to an hour.
After the hour, take handfuls of the sliced vegetables and squeeze out any excess water. Set the squeezed vegetables in a separate bowl. DO NOT THROW AWAY THE VEGETABLE WATER.
Heat oven to 420F. Line baking sheet with parchment paper and spray with olive oil.
In the bowl with the vegetable water, make the batter. Add all ingredients to the bowl and mix. If the mixture is too thick, slowly add more water until you reach a consistency like pancake batter
Add the squeezed vegetables to the batter and mix to combine.
Add mixture to the greased baking sheet in one uniform layer and as flat as possible.
Bake for 40 minutes or until golden brown and crispy.
While the tart is baking, make the sauce. Combine all ingredients together. Adjust for salt or sour.
Once tart is done, allow to cool for 30 minutes before cutting. Serve with sauce. Buon appetito!