Preheat oven to 350°. Separate the onion slices into rings and spread over the bottom of the Dutch oven or other oven-safe pan or large skillet. Add the ribs--fat side up--in a single layer, more or less.
In a small bowl stir together the next six ingredients (Dijon through garlic powder). Spread the mixture evenly over the ribs. Cover. If using a roasting pan or skillet, cover tightly with foil.) Bake for 2 hours. Uncover and continue to bake for another 30 minutes. Let the ribs rest in the pot for 5 minutes or so.
When the ribs have rested, carefully remove to a platter. Remove all but 4 T. of drippings from the pot/pan, leaving as many of the onions as possible. Over medium low heat whisk flour into the drippings. Cook, stirring constantly for a couple of minutes. Add in ⅓ of the cider. Stir constantly working to scrape up any browned bits, continue with the rest of the cider. Whisk in the chicken broth and vinegar. Bring to a boil and turn the heat to low. Continue to cook for a few minutes until the gravy is the consistency you prefer. Taste for salt and pepper, adjusting if necessary.
