How to Make It
Whisk together mayonnaise, 1 1/2 tablespoons mustard, oil, lemon juice,1/2 teaspoon salt, and 1/2 teaspoon pepper in a medium bowl. Add brussels sprouts; toss to coat. Let sit at room temperature up to 2 hours (dressing will slightly soften brussels sprouts).
Working with 1 at a time, place pork cutlets between 2 large sheets of plastic wrap; use a meat mallet or rolling pin to pound cutlets to an even 1/8-inch thickness. Season both sides of pounded pork with remaining 2 teaspoons salt and remaining 1 teaspoon pepper; set aside.
Working in 2 batches, process pretzels in a food processor to coarse crumbs, with the largest pieces about the size of Rice Krispies, about 15 seconds. Pour pretzel crumbs into a large baking dish. Place flour on a large plate. In a wide shallow bowl, whisk together eggs and remaining 2 tablespoons mustard until thoroughly combined.
Dredge pork cutlets in flour, 1 at a time, shaking off any excess. Transfer to bowl with beaten egg mixture, and turn to coat, allowing excess to drip back into bowl. Coat both sides with pretzel crumbs, pressing firmly to adhere and thoroughly coat.
Preheat oven to 200ºF. Set a wire rack over a baking sheet, and place it near the stovetop. Pour oil to depth of about 1/4 inch in a 12-inch heavy-bottomed skillet. Heat oil over medium until a pretzel crumb dropped in sizzles, browns, and then stops sizzling within 30 seconds. Carefully add 2 tablespoons butter, and melt. Add 1 or 2 cutlets to hot oil, and cook until crisp and deep brown, 2 to 3 minutes per side. Using tongs, transfer cooked pork to wire rack; keep warm in preheated oven. Repeat procedure with remaining cutlets, adding remaining 2 tablespoons butter halfway through frying. (Adjust heat between batches to maintain temperature as needed.)
Toss the pecorino into the brussels sprout slaw, and place a portion on top (or slightly to the side) of each schnitzel. Shave more pecorino on top, and serve immediately.