Preheat oven to 325F. Thoroughly combine the pepper, salt, cayenne, paprika, thyme, and mustard. Melt butter in non-stick frypan and add onions, celery, green pepper, garlic and seasonings. Saute over medium high heat, stirring occasionally, until onions are translucent, about 3-5 minutes. Do not brown. Cool mixture. Meanwhile, make deep slits in the pork roast. Place roast in roasting pan. Stuff the slits with the vegetable mixture. Rub the remainder all over the roast. Cook uncovered at 325F for 1 hour 15 minutes. Add potatoes and cook covered for 1 hour 10 min. Uncover roast and cook at 425 for 20 minutes to brown. Watch roast carefully at this point that the drippings do not scorch. Remove from oven and roasting pan and let rest for 15 minutes under foil. For gravy, deglaze the roasting pan. Melt butter in saucepan, add 1 tbsp flour. Cook over low heat, stirring for 4 minutes. Add drippings from roasting pan. Stirring constantly, continue cooking gravy until thickened. Taste and adjust salt and pepper if necessary. Enjoy!