Preheat oven to 325 degrees F.
Heat the oil in an ovenproof pot set over medium-high heat. Season the beef with salt and pepper, brown on all sides, remove to a plate. Add the onions to the pot and cook until tender, about 5 minutes. Mix in the garlic and flour and cook 2 minutes more. While stirring, slowly mix in the stock and then the tomato sauce, mustard, rosemary and bay leaf. Return the beef to the pot, cover, place in the oven and cook 2 1/2 hours, or until very tender.
When the beef is almost done, boil the potatoes until very tender. Drain well and thoroughly mash. Add the milk, butter, salt and pepper. Use an electric beater to whip until very light in texture.
When tender, remove beef from the sauce, slice and arrange on plates. Top with sauce and serve with the potatoes.