In an small bowl blend together Dijon mustard, crushed black pepper, salt, and thyme. Rub mixture over entire beef roast.
Preheat oven to 425 °F. Set up a oven safe deep skillet and heat oil and mushrooms for 5 minutes.
Scrap mushrooms to the side and place in beef roast and then red potatoes. Place 2 tbsp. of butter on top of roast and put skillet in oven uncovered.
Roast for 45-60 minutes. Use a meat thermometer to determine your temperature and how rare you want your meat. We like medium rare which is 130-135 °F.
When roasted, remove beef and mushrooms from skillet and let meat rest for 10 minutes before slicing.