Step 1: If using a block of part-skim, low-moisture mozzarella, cut it into 1/2-inch thick by 3-4 inch long sticks. If using string cheese, simply unwrap them. Arrange the mozzarella sticks on a parchment-lined baking sheet.
Step 2: Place the baking sheet with mozzarella sticks into the freezer for at least 30 minutes to firm them up.
Step 3: Set up your breading station: In the first shallow dish, place the all-purpose flour. In the second, whisk the eggs with the milk. In the third, combine panko breadcrumbs, Parmesan, garlic powder, onion powder, oregano, salt, and pepper.
Step 4: Double bread the mozzarella: Dredge each partially frozen stick first in flour, then egg, then seasoned panko. Repeat the entire flour-egg-panko process for each stick to ensure a thick coating.
Step 5: Arrange the double-breaded mozzarella sticks back on the parchment-lined baking sheet. Return them to the freezer for at least 2-4 hours, or preferably overnight, until completely solid.
Step 6: In a deep, heavy-bottomed pot, pour about 2-3 inches of vegetable oil. Heat over medium-high heat to 350°F (175°C).
Step 7: Carefully drop 3-4 frozen mozzarella sticks into the hot oil. Fry for 2-3 minutes, turning occasionally, until golden brown and crispy. Remove with a slotted spoon to a paper towel-lined plate, immediately sprinkle with salt, and serve with warm marinara sauce. Repeat with remaining sticks, ensuring oil returns to temperature between batches.