Place the sliced onions (I think that’s the secret ingredient) and the thyme at the bottom of a large roasting pan.
Pour ½ cup oil over the onions and thyme and place the roast on top of this.
Mix all the other ingredients (except the potatoes) in a small glass bowl and with the palm of you hand rub this mixture into the meat into every little nook and cranny. Place potatoes into the roasting dish with the meat.
Roast, uncovered, for 40 minutes on 400 Deg F. then reduce heat to 350 Deg F and roast, covered for a further 1 ½ hours . Keep basting the roast.
If you feel the roast is not brown enough, remove the covering on roast after 1 hour and roast it uncovered for a further 30 minutes until brown.
If you prefer your meat well done, place the cover back onto the roast, reduce the heat to 325 Deg F and roast for a further 30 - 45 minutes.
Because this cut of meat doesn't mind low and slow cooking it's a great cut for Shabbos.