Preheat oven to 450 degrees F. Set roast bone-side down in a roasting pan. Brush with mustard; season with salt and pepper. Roast 20 minutes. Lower oven to 325 degrees F. Cook roast to desired doneness (see Note), allowing approximately 20 minutes per pound for rare, 25 for medium rare, and 30 for well done.
Meanwhile, place oil in a large skillet set over medium heat. Add onions, mushrooms and garlic. Cook until the mushrooms are tender and moisture seeping from them evaporates, about 8 minutes. Mix in flour and herbs and cook 2 minutes more. While stirring, slowly mix in the stock. Bring to a simmer and cook sauce until lightly thickened. Remove from heat and set aside until needed.
When beef is cooked, transfer to a plate, tent with foil and rest 10 minutes. Skim fat from roasting pan. Set pan on over medium-high heat. Add brandy or wine, bring to simmer and reduce by half. Pour mushroom sauce into the pan and bring it to simmer. Mix in the cream; season sauce with salt and pepper. Slice beef and serve with sauce.
Note: The accurate way to test a roast for doneness is to use an instant-read meat thermometer, keeping in mind the meat will continue to cook once out of the oven and resting. For rare beef, remove from roast from the oven when temperature in the centre is 125degrees F to 130 degrees F. For medium-rare it should be 130 degrees F to 135 degrees F. Medium beef should be 140 degrees F, and well-done 150 degrees F.