Heat a medium pot over medium-high heat, and add the oil, white parts of the scallions, and the ginger. Fry for a few minutes, until the scallions are wilted and the ginger is beginning to crisp around the edges. Then add the broth and the lard, if using. Bring to a boil, then cover and simmer over low heat.
To reheat the char siu, place on a sheet of oil on a sheet pan. Lightly wrap the foil around the char siu to make a foil packet. Bake at 325°F/160°C for 15 minutes. Meanwhile, bring a separate pot of water to a boil for the rice noodles.
Boil the noodles for 60 seconds, swishing them with chopsticks to loosen them. Don’t overcook them—they are already fully cooked; they just need time to loosen up and heat through. If using dried noodles, follow package instructions and boil until the noodles are just tender.
Divide the green parts of the scallions between 2 soup bowls along with a drop of sesame oil and a dash of white pepper in each bowl, and then add the noodles on top.
Season the stock with salt to taste, then add the choy sum. Simmer for 90 seconds, until tender. Pour the stock on top of the noodles. Slice the char siu and add it on top. Garnish with the chopped cilantro if using, and serve.