Sauté the aromatics. Heat 3 Tbsp olive oil over medium heat in a large non-reactive pot. Add diced onion, then cook, stirring occasionally, until softened and lightly browned, about 5-7 minutes. Add 2 cloves of chopped garlic, then cook for 1 minute more.
Add the tomatoes. Add 2 (28oz) cans DeLallo San Marzano Style Crushed Tomatoes. When the sauce begins to simmer, add ¼ cup basil leaves (whole or chopped) and chopped oregano leaves. Season with 1 tsp kosher salt then simmer for 20 minutes. Taste and adjust the seasoning as needed. For a slightly sweet marinara sauce, stir in 1-2 teaspoons of sugar.
While the sauce is simmering, make the zucchini. Setup 2 bowls for dredging. In the first, whisk together 1 large egg and ¼ cup milk. In the second, combine 1 cup of all-purpose flour, ½ cup grated Pecorino Romano, 1 tsp onion salt, ½ tsp kosher salt, and ¼ tsp black pepper. Next, dredge the zucchini rounds first in the flour mixture, then the egg mixture, then back to the flour mixture. Be sure to coat thoroughly on all sides during all steps. Transfer to a wire rack, then repeat with remaining zucchini.
Fry the zucchini. Heat until neutral oil in a large skillet to come up about ½-inch. Set it over medium-high heat until it reaches about 375F. Working in batches, carefully lower a few of the zucchini into the hot oil. Cook for 1-2 minutes per side, or until golden brown. Immediately transfer to a wire rack and sprinkle with flaky sea salt.
Finish, then serve! Transfer the fried zucchini to a serving bowl, then sprinkle with fresh lemon zest, grated Pecorino Romano, and whole or thinly sliced basil leaves. Enjoy immediately with marinara sauce!