Cut the beef into six to eight 10-ounce portions and truss them in butcher's twine. Heat the oil in a large pot over medium-high heat. Season the beef with salt and pepper. Working in batches, sear each piece on all sides, about 5 to 7 minutes each. Remove the beef from the pot and add the chunks of carrots, celery and onions; cook over high heat until browned, 10 to 15 minutes. Add the diced tomatoes, tomato paste, rosemary, thyme and garlic and cook for 2 to 3 minutes.
Return the beef to the pot. Cover with the chicken stock, red wine and parsley stems. Cover the pot and cook until the roast is tender, 3 to 3 1/2 hours.
Bring a pot of salted water to a boil. Cook the baby carrots, fennel, onions and peas to al dente, 3 to 5 minutes each. Shock in ice water to stop the cooking process and set aside.
Remove the beef from the pot and set aside. Remove the butcher¿s twine from the beef. Simmer the remaining liquid in the pot until reduced by half, 30 to 45 minutes. Strain the liquid, discard the solids and reserve for serving.
For the mashed potatoes:
Cover the potatoes in a pot with cold salted water. Bring to a boil, lower to a simmer and cook until the potatoes are fork-tender. Drain the potatoes and put them through a ricer. While the potatoes are still hot, whip in the butter and add the creme fraiche to taste. Season with salt and pepper.
For the horseradish gremolata:
Combine the parsley, horseradish, oil, lemon zest and lemon juice in a medium bowl. Season with salt.
Heat up the reserved pot roast sauce in an oven-safe pot. Add the beef to the sauce and cook in the oven until hot and fork-tender, 15 to 20 minutes.
Add the reserved baby carrots, fennel, onions and peas to the pot and cook for 5 minutes.
Pour the sauce, pot roast and vegetables over the mashed potatoes and top with the horseradish gremolata.