Preheat the oven to 300 F.
Heat the oil in a heavy large pot over high heat.
Sprinkle the beef all over with salt and pepper.
Add the beef to the pot and cook until brown all over, about 12 minutes. Transfer the beef to a plate.
Reduce the heat to medium and add the shallots to the pot.
Sauté until the shallots are golden, about 2 minutes.
Add the carrots, celery, and garlic and sauté for 3 minutes.
Add the wine and vinegar and simmer until the liquids are reduced by half, stirring to scrape up any browned bits on the bottom of the pot, about 5 minutes.
Stir in the beef stock, thyme, rosemary, bay leaves, and juniper berries. Return the beef to the pot.
Bring to a simmer, then cover the pot and transfer it to the oven.
Braise until the beef falls apart with a fork, about 3 hours.
Remove from the oven and let the beef rest uncovered in the braising liquid for 15 minutes.
Transfer the beef to a carving board and cut into thick slices or pull the beef apart into large chunks.
Arrange the beef on a platter, spoon some braising juices over the beef, and serve.