Cut tenderloin crosswise into 4 equal pieces.
Place pieces cut side up on a cutting board, cover with plastic wrap and pound with a meat pounder or heavy cast iron pan until 1/4-inch thick. Pat dry.
Place flour in a shallow dish. Place eggs in a second dish. In a third dish, combine Panko crumbs, Parmesan cheese, parsley, lemon zest, salt and pepper.
Dredge cutlets in flour, dip in eggs, and then coat in Panko crumb mixture, pressing the Panko crumbs into the cutlets to make a nice thick layer.
Heat the oil in a 12-inch nonstick skillet over medium-high heat. When oil sizzles when you drop a bread crumb in, add cutlets. You may need to do them in 2 batches. You don't want them crowded.
Cook cutlets about 3 minutes per side, or until cooked through. If they are browning too quickly, turn heat down some. Transfer to a paper towel-lined plate.