Place beef, onions, vinegar, water, 1 tablespoon salt, 1 tablespoon black pepper, sugar, cloves, and bay leaves in a large pot. Cover and refrigerate for 2 to 3 days, turning meat daily. Remove meat from marinade and pat dry with paper towels, reserving marinade.
Season flour to taste with salt and black pepper in a large bowl. Sprinkle flour mixture over beef. Heat vegetable oil in a large Dutch oven or pot over medium heat; brown beef on all sides, about 10 minutes. Transfer the beef to a slow cooker.
Pour the reserved marinade over the beef and cook on Low setting for 6-8 hours. When beef is fork tender, remove from the slow cooker to a plate and slice.
Strain the liquid from the slow cooker and make a gravy by heating that in a small pot and adding cornstarch.
Serve with potatoes or potato dumplings.