Season the beef. Pat the beef dry with paper towels. Season generously with salt and pepper, tossing to coat.
Sear the beef. Heat 2 tablespoons of oil in a large skillet over medium-high heat. Sear the beef in batches, if needed, until deeply browned on all sides. Transfer the beef to the slow cooker.
Sauté the onions and garlic. Add the remaining tablespoon of oil to the skillet and sauté the onions until softened and lightly browned, scraping up any browned bits from the bottom of the pan. Add the whole garlic cloves and cook for another minute, until fragrant.
Build the cholent. Transfer the onions and garlic to the slow cooker with the beef. Add the soaked beans, pearl barley, potatoes, and nestle in the marrow bones. Season with the paprika, ground ginger, salt, and pepper.
Add the stock and cook. Pour in 4 cups of stock or water. The ingredients should be mostly submerged. Cover and cook on LOW for 12 hours, preferably overnight.
Skim and serve. When the cholent is finished, use a spoon to skim excess fat from the surface. You can also refrigerate it and remove the solidified fat from the top before reheating.
Serve the tender beef with the beans, barley, potatoes, and rich broth.