Butterfly pork pieces; flatten between parchment to an even 1/4-inch thickness.
Salt pork; coat in flour, beaten egg, then combined panko and cracker crumbs.
Heat oil in a deep skillet to 350°F; keep skillet no more than half full.
Fry 2 cutlets at a time 2 to 3 minutes per side, maintaining 340°F to 350°F, until crisp and centers reach 145°F.
Drain on a rack 3 minutes to keep bottom crust firm.
Spread buns with mustard; add one cutlet, 4 pickles, and onion each.
Close without trimming overhanging crisp edges.