Season both sides of the pork chops liberally with the salt and pepper. In a very hot frying pan, melt 2 tablespoons of the butter. Sear the pork chops for 2 to 3 minutes per side, until they have a nice brown color. Transfer the chops to a slow cooker.
Add the remaining 2 tablespoons of butter and the chopped onion to the pan. Sauté for 8 to 10 minutes, until the onion is translucent and slightly browned. Add the mustard and vinegar, whisking to incorporate and working to release the browned bits from the bottom of the pan so that they mix into the sauce. Pour the sauce over the pork chops in the slow cooker.
Cook on high for 3 hours or on low for 6 to 7 hours, until the chops are fork-tender. Serve garnished with fresh parsley, if desired.
Notes
Instead of step 3 using the slow cooker, I placed my skillet in the oven at 350° for 15 to 20 minutes and it was delicious!Leftovers will keep refrigerated for up to 5 days or frozen for up to 5 months.