Pat the roast beef dry and remove any tendons. Peel and finely chop the onions. Mince the garlic cloves. Pour sunflower oil into a large baking dish or roasting pan. Add the roast beef, chopped onions, garlic, allspice grains, juniper berries, bay leaves, peppercorns, rosemary (sprigs or ground), and thyme (sprigs or ground). Turn the meat to coat it in the oil and spices. Pour the red wine over the meat and vegetables.
Cover the baking dish with a plate and refrigerate for 24 hours, turning the meat 1-2 times during this period.
Remove the roast beef from the marinade, discarding the vegetables and spices, and pat the meat dry.
Peel the onions, celeriac, and carrots. Cut them into approximately 1 cm cubes of similar size. Peel the garlic cloves and add them to the vegetables. Set aside.
Pour sunflower oil into a large roasting pan or heavy-bottomed pot and heat over medium-high. Brown the roast beef on all sides, about 8-10 minutes total. Remove the meat and set aside.
In the same pan over medium heat, roast the prepared vegetables and garlic for 3-5 minutes, stirring frequently. Stir in the tomato paste and roast for 3 minutes. Deglaze the pan with the red wine, scraping up any browned bits. Return the roast beef to the pan and pour in the beef stock. Add the allspice grains, juniper berries, bay leaves, peppercorns, rosemary, and thyme.
Reduce heat to low, cover with a lid, and simmer for 2 hours, turning the meat once halfway through. Season with salt and pepper to taste. If you have more time, simmer for 3 hours instead.
Remove the roast beef from the pan and keep it warm, loosely covered with foil.
Pour the braising liquid through a sieve or colander into a large saucepan, pressing gently with the back of a spoon to extract as much liquid as possible. Return the roast beef to the original pan and pour the strained braising liquid back over it.
Reduce heat to low and simmer for 1 more hour.
Meanwhile, prepare your desired side dishes, such as potatoes with red cabbage, fried potatoes with green beans, or potatoes with cauliflower, broccoli, and Brussels sprouts.
Remove the roast beef from the pan and wrap it loosely in aluminum foil. Let it rest for 10 minutes, then slice it.
Pour the braising sauce into a serving bowl.
Serve the sliced roast beef with the sauce and side dishes.
If you prefer a thicker sauce, reduce it on the stovetop by simmering uncovered until it reaches the desired consistency, or stir in a sauce thickener according to package directions.