Heat the oil in a large pot over high heat. Season the beef all over with salt and pepper. Working in batches if necessary, sear the beef on all sides for 3 to 4 minutes, until browned. Transfer to a slow cooker and add the stock, water, soy sauce, onions, garlic, ginger, and star anise. Cook on low for 6 hours, until the beef is very tender. (Or, simmer everything in the pot over a very low flame for 2 to 3 hours.) When the beef is nearly ready, prepare the noodles according to package instructions. Add the bok choy to the soup, and simmer for about 10 minutes, until tender. Season to taste with salt (if it needs any) and plenty of black pepper. Divide the noodles among 8 large bowls. Ladle the broth, along with a generous amount of beef and bok choy, into each bowl. Top with any of the garnishes you like.