For the most succulent schnitzel, pound the tenderloin as thin as possible without tearing it. Toasting the panko ensures a golden brown crust without frying.
Preheat oven to 350 degree. Spread panko on a rimmed baking sheet. Bake until golden, 10 to 12 minutes, tossing once halfway through. Set panko aside, and increase oven temperature to 450 degrees.
Cut pork crosswise into 4 equal pieces. Without cutting all the way through, split each piece in half horizontally, opening it like a book. One at a time, place a piece of pork inside a resealable plastic bag. Using a meat mallet or the bottom of a heavy pan, pound pork into a very thin cutlet.
Place flour, eggs, and toasted panko in separate shallow bowls; season each with salt and pepper. Dredge pork in flour (shake off excess), dip in eggs (allowing excess to drip off), and dredge in panko, pressing to adhere; place on rimmed baking sheet. Bake, without turning, until pork is cooked through, 10 to 12 minutes, rotating sheet halfway through.
Meanwhile, in a small saucepan, melt 1 tablespoon butter over medium. Add shallot, and cook until softened, 3 to 4 minutes. Add wine, and simmer until almost evaporated, about 4 minutes. Remove from heat; stir in lemon juice and remaining 2 tablespoons butter; season with salt and pepper. Serve pork with sauce on the side.