Combine bell pepper, garlic, onion, celery, salt, and cayenne pepper; make slits in the pork roast and stuff the mixture into the slits.

Rub remaining onion mixture on the outside of the roast.

Heat cooking oil in a large cast-iron skillet with a lid; brown roast well on all sides. Add water, cover, and cook over low heat until roast is fork-tender, about 3½ hours.

Have plenty of hot cooked rice on hand for serving.