In a small bowl combine the onions, celery, bell peppers, butter and seasonings, mix well
Place roast in a large roasting pan, fat side up. With a large knife make 6 to 12 deep slits in the meat, to form pockets, down to a depth of about 1/2 inch from the bottom; do not cut all the way through
Fill the pockets to their depths with the vegetable mixture, reserving about 1 tablespoon of the vegetables to rub over the top of the roast
Bake uncovered at 300 degrees F until a meat thermometer reads about 160 degrees F for medium doneness, about 3 hours. For rarer roast, cook until thermometer reads 140 degrees F. Serve immediately topped with some of the pan drippings if you like.
