Chop 5-7 garlic cloves and 1 onion. Place 900-1200g beef in a large bowl. Cover with 1 liter kvass and add the chopped garlic, chopped onion, 1.5-2 tablespoons salt, 3-4 black peppercorns, 2-3 bay leaves, 2 cloves, and Provence herbs to taste. Mix well. Refrigerate for at least 2-4 hours, preferably overnight.
Remove the marinated beef from the bowl and rub it thoroughly with Provence herbs and ground black pepper to taste. Add enough oil to coat the bottom of the slow cooker and set to browning mode at medium-high heat. Place the beef in the slow cooker.
Brown the beef on one side until light golden (about 3-4 minutes), then turn it over and brown all remaining sides (about 3-4 minutes more).
Switch the slow cooker to slow cooking/stewing mode and set the timer to 1.5 hours. Close the lid and let it cook until the meat is tender.
Remove the meat from the slow cooker and let it cool at room temperature for about 30-45 minutes. Transfer to the refrigerator and chill for 1-2 hours. Serve.