How to Make It
Combine first 4 ingredients and 1⁄3 cup of the salt in a large bowl; whisk until salt and sugar dissolve. Add pork chops, submerging completely; cover and chill 1 1⁄2 hours. Remove from brine; discard brine. Pat dry, and sprinkle with pepper and remaining 1 1⁄2 teaspoons salt.
Heat oil in a large skillet over medium-high. Cook pork chops until browned, 3 to 4 minutes per side. Remove from skillet. Add onions; cook, stirring occasionally, until softened, about 6 minutes. Add apple cider. Cook until reduced slightly, 2 minutes. Transfer onion mixture to a 6-quart slow cooker; stir in broth and mustard. Place pork chops on top of onion mixture. Cover; cook on LOW until pork is tender, 4 hours.