Make an herb rub for the meat by mixing together the mustard, rosemary, thyme, teaspoon of salt, teaspoon of pepper and 1 tablespoon of olive oil in a small dish. Pat the beef dry and rub on all sides with the herb mixture.
In a heavy pot or Dutch oven, heat 2 tablespoons of olive oil, add the beef and sear until browned on all sides. Remove the beef from the pot.
Add the remaining oil to the pot along with the chopped onions, carrots, and garlic. Season with salt and pepper. Cook, stirring frequently until tender, about 10 minutes. De-glaze by adding the sherry and scraping up the brown bits from the bottom of the pan.
Stir in the beef stock and return the meat to the pot. Add the six additional carrots and the baby potatoes to the pot. Cover and cook in a 350 degree oven for four hours, turning the beef over halfway through and adding additional stock if needed. When done, remove the roast, larger carrot pieces, and potatoes. Keep warm under aluminum foil.
Blend the sauce with a stick blender, regular blender or food processor. I like mine to have some texture so I leave it a bit chunky. Taste the sauce and correct seasoning with salt and pepper if necessary.
Pull the meat apart with two forks or cut into serving sizes and return the roast, carrots, and potatoes to the pot.
Serve warm with gravy.