Heat the oven to 400ºF. Season the beef generously with salt and pepper.
Set a large oven-proof pot over high heat. Add the oil. Cook the beef for 5 minutes on each side, or until deeply browned. Transfer to a plate.
Reduce the heat to medium. Add the onions and garlic to the pot and cook 5 minutes, or until soft and translucent. Add the mushrooms, rosemary, peppercorns, and tomato paste; cook for 1 minute. Stir in the wine, scraping up any browned bits from the bottom of the pot. Return the beef to the pot; add the beef stock and water; bring to a simmer. Cover the pot and transfer to the oven.
After 1 hour, poke the potatoes several times with a fork. Rub them with the vegetable oil and sprinkle them with sea salt. Place them in the oven, directly on the rack.
After another hour, check the potatoes for doneness – the skin should be crispy and the insides should feel tender. When the potatoes are done, remove them and the pot roast from the oven. Use two forks to shred the pot roast; it should be very tender and fall apart easily.
Stir in the balsamic vinegar and let the shredded meat rest in the pan juices for 5 minutes.
Meanwhile, prepare the gremolata by mixing together the lemon zest, parsley, and garlic cloves.
To serve, cut a slit in the center of each potato, lengthwise. Gently squeeze the opposite ends toward each other to help it open up. (If necessary, you can use a spoon to scoop out some of the inside of the potato.) Spoon about 1/4 cup of pot roast into each potato. Top with gremolata and sour cream, if desired.