Slice the zucchini. Cut each zucchini lengthwise into ½-inch-thick planks (3–4 per zucchini).
Salt and rest. Lay the planks on a clean towel in a single layer. Sprinkle with ½ teaspoon kosher salt and rest for 10 minutes. Pat completely dry.
Preheat the grill. Insert the grill grate. Select GRILL, set temperature to MAX, set time to 8 minutes. Press START. Wait for the ADD FOOD prompt (about 6 minutes).
Season. While the grill preheats, toss the dried planks with olive oil, remaining ¼ teaspoon kosher salt, pepper, garlic powder, and Italian seasoning.
Grill side one. Lay planks perpendicular to the grates. Close the hood and grill undisturbed for 3 minutes.
Grill side two. Flip each plank with tongs. Close hood and grill 2–3 more minutes until grill marks are deep and the zucchini just bends.
Serve immediately. Transfer to a plate, squeeze with lemon juice, sprinkle with parmesan if using, and finish with flaky sea salt.