In a medium bowl, stir together the brown mustard, brown sugar, maple syrup, yellow mustard, minced onion, apple cider vinegar, soy sauce, pepper, and garlic. Pour half of the marinade in a gallon zip-top plastic bag, and reserve the remaining marinade. Add the pork chops to bag and mash them around to fully coat with the marinade. Seal the bag and refrigerate for at least 2 hours.
Preheat grill or grill pan to medium-high heat.
Remove the pork chops from the bag and sprinkle with salt. Place the pork chops on the hot grill rack coated with cooking spray. Sear each side for 1 minute with the grill lid closed, then reduce the heat to medium and cook the chops about 3 minutes on each side. (For the grill pan, I cooked each chop for about 3 to 4 minutes on each side without the initial sear.) Set the cooked chops on a plate and cover with aluminum foil. Allow to rest for 5 to 10 minutes, then serve with the reserved mustard marinade.