Heat olive oil over medium heat in a large non-reactive pot. Add diced onion, then cook, stirring occasionally, until softened and lightly browned. Add chopped garlic, then cook.
Add DeLallo San Marzano Style Crushed Tomatoes. When the sauce begins to simmer, add basil leaves (whole or chopped) and chopped oregano leaves. Season with kosher salt then simmer. Taste and adjust the seasoning as needed.
Whisk together large egg and milk. In the second, combine all-purpose flour, grated Pecorino Romano, onion salt, kosher salt, and black pepper. Next, dredge the zucchini rounds first in the flour mixture, then the egg mixture, then back to the flour mixture.
Heat until neutral oil in a large skillet to come up about ½-inch. Working in batches, carefully lower a few of the zucchini into the hot oil. Cook. Immediately transfer to a wire rack.
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!