Trim and very thinly slice 1 1/2 pounds zucchini (about 5 cups) and 1/2 medium white or yellow onion (about 1 cups). Place in a colander, sprinkle with 2 teaspoons of the kosher salt, and toss to combine, making sure all the zucchini slices are separated. Place the colander in the sink or set over a large bowl and let sit for 45 minutes to drain.
Arrange a rack in the middle of the oven and heat the oven to 375ºF. Coat a 10-inch round baking dish, cast iron skillet, or high-sided ovenproof skillet with olive oil. Push down on the zucchini and onion to press out any remaining water or squeeze handfuls with your hands, then pat dry with paper towels.
Place 2 large eggs in a large bowl and whisk to combine. Prepare the following, adding each to the bowl as it is completed: Finely grate 2 garlic cloves. Pick the leaves from 2 fresh basil sprigs and thinly slice until you have 3 tablespoons. Finely grate 1 ounce Parmesan cheese until you have 1/2 cup, or measure out 1/3 cup store-bought grated.
Add 3/4 cup whole milk, 1/4 to 1/2 teaspoon red pepper flakes, 1/4 teaspoon black pepper, and the remaining 1/2 teaspoon kosher salt. Whisk until combined. Add 1 cup all-purpose flour and 1/3 cup cornmeal. Stir until the batter is smooth. Add the zucchini and onion and fold with a flexible spatula until combined.
Transfer to the baking dish and spread into an even layer. Sprinkle with a pinch of flaky salt and drizzle with 2 tablespoons olive oil.
Bake until golden-brown around the edges and set in the middle, 35 to 40 minutes. Let cool for 15 minutes before slicing and serving. Garnish with more grated Parmesan and sliced fresh basil leaves before serving
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