Step 1

Line a rimmed baking sheet with a layer of paper towels. Trim and slice 2 1/2 pounds zucchini lengthwise into 1/2-inch wide planks. Place on the paper towels in a single layer. Sprinkle the zucchini with 1/2 teaspoon of the kosher salt, then cover with another layer of paper towels.

Step 2

Finely grate 6 ounces Parmesan cheese (about 1 1/2 packed cups). Place 1 cup in a medium bowl. Shred 12 ounces mozzarella cheese (about 3 cups), add to the 1 cup Parmesan, and toss to combine.

Step 3

Transfer the remaining 1/2 cup Parmesan to a wide, medium bowl. Add 2 cups seasoned breadcrumbs, 1/2 teaspoon of the kosher salt, and 1/2 teaspoon black pepper, and stir with a fork until well combined. Whisk 4 large eggs and 1/2 teaspoon of the kosher salt together in a second wide, medium bowl. Whisk 1 1/2 cups all-purpose flour and the remaining 1/2 teaspoon kosher salt together in a third wide, medium bowl.

Step 4

Pat the zucchini planks dry with more paper towels and stack on a large plate. Wipe the baking sheet dry and fit with a wire rack. Working with 1 zucchini plank at a time, coat it into the flour mixture, then tap off any excess flour. Dip in the egg mixture. Let the excess egg drip back off into the bowl, then dip the zucchini in the breadcrumbs, pressing to adhere. Place on the wire rack. Repeat with the remaining pieces, placing them in a single layer (they can touch).

Step 5

Arrange a rack in the middle of the oven and heat the oven to 425ºF. Meanwhile, fry the zucchini.

Step 6

Line a second baking sheet with paper towels. Heat 1 cup of the olive oil in a large frying pan over medium-high heat until shimmering. Test the oil heat by sprinkling some breadcrumbs in it: If it sizzles immediately, the oil is ready. Working in batches, add 3 to 4 breaded zucchini to the pan and fry until deep golden brown, 2 to 2 1/2 minutes per side. Transfer to the paper towel-lined baking sheet. Repeat with the remaining zucchini, adding more olive oil to the pan as needed so that the zucchini are covered halfway in oil.

Step 7

Spread 3/4 cup of the marinara sauce in the bottom of a 2-quart baking dish (such as an 8 or 9-inch square). Top with 1 layer of zucchini planks, cutting the planks to fit in the gaps as needed. Spread 1 cup of the marinara sauce over the planks. Sprinkle with 1 1/2 cups of the mozzarella cheese mixture. Top with one more layer of zucchini. Spread the remaining 1 1/2 cups marinara over the zucchini, then top with the remaining cheese mixture.

Step 8

Coat a piece of aluminum foil large enough to cover the baking dish with olive oil. Place the foil oil-side down on the baking dish and tightly cover. Bake until the zucchini is tender when poked with a paring knife or cake tester, 40 to 45 minutes. Uncover and turn the oven to broil. Broil until the cheese on top is golden brown, 2 to 3 minutes. Let cool 20 minutes before slicing.