- Preheat the oven to 135 ºC (275 ºF).
- In a saucepan, sauté the onions and garlic in half the butter. Place in a large baking dish. Pour the broth over the onions.
- Pat the roast beef dry and rub with the remaining butter. Brush with the mustard. Season with salt and pepper.
- Place the roast in the baking dish, bone side down.
- Cook the roast for about 1 hour 30 minutes, 25 minutes per pound or until the roast internal temperature reads 54 º C (130 º F). Remove the roast from the broth and cover with aluminum foil. Let rest for 10 minutes before untying and slice thinly.
- Serve with the broth with the onions and good mashed potatoes.