Allow the ribeye to come to room temperature at least half an hour before roasting.
Preheat oven to 250°C, or as hot as your oven will go.
In a bowl, combine spice and olive oil to form a paste. Cover the roast with the paste and set aside.
Arrange Spanish onions and whole shallots on an oven tray. Add 2 cups of water to tray, or enough to just cover the onions.
Set a roasting rack over the onions in the oven tray, then sit the roast on the rack. Place roast on lowest rack in oven and cook at 250°C for 45 minutes. Then, reduce the temperature to 180°C and roast for a further 30 minutes. Check the water level in the tray and add another 1-2 cups of water if needed.
Finally, increase the temperature back up to 230°C and cook for a further 15 minutes for medium rare or 30 minutes for medium well done. (See Recipe Note for cooking temperatures.)
Take out and rest covered with foil 20-30 minutes before cutting.
Serve with onions and shallots. Pair with Perfect Roast Potatoes.
