Step 1: Prep the Zucchini. First things first, give your zucchini a good wash. Trim off the ends and then slice each zucchini lengthwise into nice 1/2-inch thick pieces. You should get about 4 slices from each medium zucchini, totaling 8 slices. Set these aside.
Step 2: Set Up Your Dredging Station. Grab three shallow bowls or plates. In the first one, put your all-purpose flour. In the second, crack the egg and whisk it up gently with a fork. In the third bowl, mix together the panko breadcrumbs, dried parsley, and that lovely grated Parmesan. This is your assembly line for coating the zucchini.
Step 3: Heat the Oil. Pour about 1 cup of vegetable oil into a large nonstick skillet—just enough to cover the bottom generously. Place the skillet over medium-low heat and let the oil warm up. You want it hot enough to sizzle when you add the zucchini, but not so hot it smokes.
Step 4: Coat the Zucchini. While the oil heats, it's time to coat those zucchini slices! Take one slice at a time. First, dredge it in the flour, making sure to shake off any extra. Then, dip it into the whisked egg, letting the excess egg drip back into the bowl. Finally, transfer the eggy slice to the breadcrumb mixture and press gently to make sure it's well coated on both sides. Repeat this process for all 8 zucchini slices. You can stack the coated slices on a plate while you work.
Step 5: Pan-Fry to Golden. Once your oil is hot and shimmering (you can test it with a tiny pinch of breadcrumbs—they should sizzle gently), carefully place 2 or 3 coated zucchini slices into the skillet. Don't overcrowd the pan, or they won't get nice and golden! Cook them for about 3-4 minutes per side until they are beautifully golden brown and crispy. The second side usually takes just a touch less time, maybe 2-3 minutes. A splatter screen is your friend here if things get lively! As they finish, transfer the cooked slices to a baking sheet. Avoid silicone mats for the broiler step later.
Step 6: Repeat and Refill. Keep pan-frying the remaining zucchini slices in batches. Add more oil to the skillet as needed between batches and make sure it heats back up before you add more zucchini. Once all the slices are cooked, turn off the stove.
Step 7: Add the First Layer of Cheese. Sprinkle some shredded fontina and mozzarella cheese evenly over each of the warm zucchini slices on the baking sheet. They'll start to get melty just from the residual heat, which is great!
Step 8: A Quick Broil #1. Turn on your oven's broiler. Place the baking sheet with the cheesy zucchini slices on the top rack, close to the broiler element. Broil for just a few seconds—literally, a few seconds!—until the cheese is mostly melted. Keep the oven door cracked open and watch it like a hawk!
Step 9: Add Sauce and More Cheese. Carefully pull the oven rack out (or remove the sheet entirely). Spoon a small amount of warm marinara sauce onto each zucchini slice over the melted cheese. Then, top each slice with the remaining mozzarella and fontina cheese, and sprinkle with extra Parmesan if you like.
Step 10: A Quick Broil #2. Return the tray to the broiler on the top rack. Broil again for just a few more seconds until the cheese is fully melted, bubbly, and maybe just starting to get a little golden-brown char in spots. Watch it NON-STOP!
Step 11: Serve Immediately! Carefully remove the baking sheet from the oven. Let them sit for just a second, then transfer these delicious cheesy zucchini stacks to plates. Serve them right away while the cheese is still gooey and the zucchini is warm. Enjoy every bite!