How to Make It
In a large enameled cast-iron casserole, combine the short ribs, rump roast, beef shank and bouquet garni. Press 1 teaspoon of sea salt into each end of the marrow bones and add to the pot, tucking them between the meats. Add enough cold water to cover the meat and bring to a boil over high heat. Reduce the heat to moderately low and simmer, skimming occasionally, for 2 hours 30 minutes. Add the turnips, carrots, radishes, leeks, parsnips, onion and celery root and cook, partially covered, until the vegetables are very tender, about 1 hour.
Using a slotted spoon, transfer the vegetables and marrow bones to a large serving platter. Transfer the meat to a carving board. Discard the string from the rump roast. Slice all the meats and arrange on the platter. Cover and keep warm.
Strain the broth through a sieve into a large saucepan; discard the bouquet garni. Bring the broth to a simmer and skim. Season with sea salt and pepper. Transfer to a pitcher. Serve with the meat, vegetables and marrow bones. Pass the fleur de sel, cornichons and mustard at the table.