Season beef with salt and pepper. Sear on both sides, then set aside.
Lightly brown the ginger. Sauté onion and garlic until soft.
Add beef and 8 cups water, or enough to cover. Bring to a boil, spooning off the foam as it rises.
Add beef base or bouillon, salt, and pepper.
Cover and simmer on medium-low for 2 to 2½ hours, until tender. Add more water as needed, then season with fish sauce, salt, or pepper.
Add potatoes and corn. Simmer for about 10 minutes.
Add green beans and cook for another 3 minutes.
Add bok choy, napa cabbage, and cilantro stems (if using). Cook for about 2 minutes, then turn off the heat.
Serve with fish sauce and calamansi on the side.