With the rack in the middle position, preheat the oven to 140 °C (275 °F).

In a skillet, brown the roast on all sides in the oil and half the butter. Season with salt and pepper. Remove the meat from the pan. Set aside.

In the same skillet, sauté the onions in the remaining butter. Add the garlic, wine and herbs. Season with salt and pepper.

Pour the content of the skillet in a roasting pan. Place a wire rack in the pan and top with the roast. Insert a meat thermometer in the centre of the piece of meat. For rare meat, cook for about 1 hour and 55 minutes or until the thermometer reads 55 °C (130 °F).

Remove the roast from the oven. Cover with aluminum foil. Let stand for 15 minutes. The internal temperature should rise to 60 °C (140 °F).

Before serving, thinly slice the roast. Serve with the onion sauce and mashed potatoes.