Season the meat with salt and pepper. Heat oil in a large pot over medium-high heat. Sear the meat until browned on both sides, then set aside.
Reduce the heat to medium, then sauté the ginger until fragrant. Add the onion and garlic, then sauté until slightly softened.
Return the beef to the pot, then add enough water to fully submerge the meat, about 8 cups. Increase the heat to high and bring the water to a near boil.
As the water heats, remove the scum that forms on the surface. Continue to do so until the water reaches a full boil.
Add beef bouillon, a generous pinch of salt, and some fresh cracked pepper to taste.
Cover and simmer over medium-low for 2 to 2½ hours, until the meat is tender. After simmering, add up to 4 cups of water to reach the desired broth level. Season with fish sauce, salt, and pepper to taste.
Add the potatoes and corn. Continue to simmer for 10 minutes, or until the potatoes are tender and the corn is cooked through.
Add the green beans, then simmer for another 3 minutes or until tender yet retain their color.
Add the bok choy and napa cabbage, then simmer for about 2 minutes until the vegetables are tender but still vibrant. Turn off the heat.
While not traditional, garnishing with cilantro stems is highly recommended for a fresh, aromatic flavor. Turn off the heat.