Cut the pork tenderloin crosswise into 4 even pieces. Cut each piece crosswise again, but do not go all the way through; you should be able to open it like a book. Pound each piece with a mallet until 1/4-inch thick and 6 to 8 inches across.
Whisk together the buttermilk, garlic powder, onion powder, paprika, sugar, 1 teaspoon salt and 1/2 teaspoon pepper in a shallow dish. Add the pork cutlets. Make sure all the pieces are coated well with the buttermilk marinade. Cover and refrigerate for at least 4 hours and up to overnight.
Set up a breading station with 3 shallow dishes: the flour in the first dish, eggs in the second and breadcrumbs in the third. Sprinkle all 3 dishes with salt and pepper.
Fill a large, high-sided skillet with about 1/2-inch oil, attach a deep fat fry thermometer and heat to 350 degrees F.
Dredge each pork cutlet in the flour first, then in the egg and finally in the breadcrumbs. Gently place into the hot oil without overcrowding the skillet. You will need to fry the cutlets in batches. When the cutlets are golden brown on one side, about 2 minutes, carefully turn and cook for another 2 minutes. Drain well on a paper-towel-lined plate.
Build the sandwiches by putting the cutlets between the burger buns and garnishing with lettuce, tomato, onion, pickle, mayonnaise and mustard. The cutlet should hang generously over the bun.
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