In a bowl, whisk together ketchup, broth, Worcestershire sauce, brown sugar, salt, chili powder, and ground mustard. Set aside.
Pat pork chops dry with a paper towel. Season both sides of the pork chops liberally with salt and pepper. Heat about 1 tablespoon of oil in a large skillet over high heat. When hot, add the pork chops to the skillet. Sear both sides of chops over high heat (about 3-5 minutes per side), and then transfer to slow cooker.
Pour sauce over the pork chops and add onion on top. Cover the slow cooker and cook until the pork chops reach an internal temperature of 145°F, about 1-2 hours on HIGH or 2-4 hours on LOW.
Serve pork chops smothered in onions and sauce, and garnished with a squeeze of lemon juice and chopped fresh parsley.