Adjust oven rack to middle position and preheat oven to 350F.
Combine 2 teaspoons of salt with the dry seasonings (onion powder, granulated garlic, paprika, marjoram, thyme, allspice, and cayenne pepper). Pat dry pork chops with a paper towel. Sprinkle each chop with about one teaspoon of the seasoning mix per side. Pat the seasonings down to make sure they stick. Set aside.
Combine the remaining seasonings with ½ a cup of all-purpose flour on a large plate or a platter. Set aside.
Heat oil in a 12-inch cast-iron pan over medium-high heat until just smoking (about 300F). One a time, dredge each pork chop in flour on both sides, shake off excess and place in the pan. Fry until deep golden brown on each side, about 3-5 minutes per side. Reduce heat to medium. Remove the chops from the pan and set aside.
You should have about ¼ cup of oil left in the pan. If less, add more oil. Add the seasoned flour left from dredging of pork chops to the pan and cook over medium heat, stirring constantly, until the roux acquires a nutty flavor and the color becomes deep golden brown, about 3-5 minutes. Add the sliced onions and continue cooking, stirring constantly, until the onions begin to soften slightly, about 2 minutes.
Slowly add the water into the roux mixture, constantly stirring until the gravy is smooth and free of lumps. Bring to simmer and cook the gravy until it begins to thicken, about 2 minutes. Add the vinegar and stir well. Add the remaining 1 ½ teaspoons of salt (plus more if needed), starting with 1 teaspoon and adjusting to taste.
Turn off the heat and add the pork chops back to the cast iron pan, nestling them such that they are fully covered with the gravy and onions.
Carefully cover the pan with aluminum foil -use oven mitts as the pan will be hot – and transfer to the pre-heated oven. Bake, covered, until the chops are fully tender, about 1 ½ hours.
Carefully transfer the chops to a serving dish (the chops will be delicate and may fall apart). Pour the gravy over the chops and serve.