Butterfly the chops: Lay them flat on a cutting board and, holding your knife parallel to the board, cut through the meat along the side of each chop until you reach the bone. This cut should evenly divide the meat of each chop into 2 equal flaps, which remain attached to the bone. Spread each chop out in the shape of a butterfly.
Place the chops between sheets of wax paper, and pound each flap with a mallet or the side or back of a heavy cleaver. Pound until each flap is an even 1/4-inch thick.
Mix together the 1/4 cup rice-wine vinegar, soy sauce, sugar and a few drops of Japanese sesame oil. Place the pounded pork chops in a wide, shallow dish and pour the marinade over them. Marinate in the refrigerator, turning occasionally, for 2 hours.
When ready to cook, heat the peanut oil in a wok, deep-fryer, or deep, wide pot to 365 degrees.
While the oil is heating, remove pork chops from the marinade and shake off liquid. Dredge chops in flour, making sure to cover all spots of the meat and bone. Then dip the chops in the beaten egg, and let the excess egg drip off. Finally, dip the chops in the panko crumbs, making sure to cover the entire meat and bone.
When the oil is hot, add the pork chops (if your frying vessel is not large enough, you should do this in 2 batches). Deep-fry until chops are golden brown on the outside, just cooked through on the inside, 3 to 4 minutes altogether. Remove and drain on paper towels.
While the pork chops are cooking, make the watercress salad: Toss together the watercress, tomato, scallion, bean sprouts, rice-wine vinegar and sesame oil. (Make sure the salad has a light taste of the sesame oil; if not, add a little more.) Season with salt.
Place each chop on a large dinner plate and season with coarse salt. Strew each chop with the watercress salad and serve immediately.
*panko crumbs are Japanese bread crumbs -- light, airy, remarkably crisp. They are available at Japanese groceries.
(Recipe courtesy of David Rosengarten, The Dean & DeLuca Cookbook, Random House, 1996)