Remove roast from the refrigerator about an hour before you plan on cooking it. Preheat oven to 450° F (230°C.)
Use a pastry brush to cover the roast in oil, then roll the roast in the pepper. You can use your hands to rub the pepper into the meat so that it adheres a bit more. Salt the roast lightly.
Warm a Dutch oven on medium to high heat on the stove. When it is quite hot, place the roast in the Dutch oven and sear it for several minutes on each side. This takes about 7 minutes for a two pound roast.
Remove the roast from the Dutch oven and set it aside. Turn the heat down and melt the butter in the Dutch oven. Turn off the heat and return the roast to the dish.
Place the dish in the oven and cook for 20 minutes. This gives me a medium rare (140°F on meat thermometer) 2 pound roast working with my oven, but you will need to adjust according to the size of your roast and your oven. When it is done to your liking, remove the roast from the oven and place it on a serving dish. Cover the roast with aluminum foil to keep it warm.
Add the Armagnac to the Dutch oven and place on low heat. Use a wooden spatula to scrape all of the meat bits off of the bottom of the dish and into the sauce. Stir in the mustard and heavy cream and warm slowly until almost boiling. Slice the beef and add any juices to the sauce as well. Season to taste with salt and pepper.