1Prep: Cut (large) onion in half, cut each half into 8 wedges. Slice mushrooms. Set aside.
2Wipe roast with damp paper towel.
Coat each side generously with coarse-ground black pepper and coarse-ground salt, and pat into roast with hands. Heat small amount of oil in dutch oven or heavy pot**, and sear meat on all sides in hot oil, getting a nice dark sear on every side. remove from pan.
** NOTE: since this needs to cook at low heat for a long time, would be perfect for crockpot if you have one...i don't
3Add butter to pan, saute onion wedges (on all sides) till brown and soft. Stir in mushroom slices, saute till soft. Return roast to pan, surround with cooked onions and mushrooms.
4Add wine and beef broth to pan, bring to a boil. Turn down heat, cover and simmer for approx. 5 hours (until meat thermometer reads 170°). Remove roast to plate to rest. Stir sauce, turn heat back up to make sure it's hot.
5Meat should be very tender; separate into large chunks and return to pan with sauce.
To serve: place several "medallions" on plate, cover with wine/onion/mushroom sauce. Serve with rice or mashed potatoes and a green vegetable.