If you're using leftover or roasted chicken or turkey, remove about 75% of the meat and reserve this to add to your soup / cháo when serving.
In a large stock pot, add the chicken or turkey, 85% of the filtered water, ginger, onion, salt and fish sauce over high heat. Once it hits a boil, reduce the heat so its at a low to medium simmer. The water should just about cover all the meat and bones. If your pot is very wide, keep in mind you may be using more water than normal, and you may need to add less in the final steps later when adjusting for consistency. Skim off any scum if necessary during the boil.
After about 90-120 minutes or when the meat has softened enough to easily be pulled off the bone, remove the poultry and let cool for about 10 minutes. Using your hands or a fork, remove the rest of the meat from the bones. Add the meat back into the pot. Discard the bones, onion, and ginger, since they've given their all to the broth at this point.
Add all the rice to the pot and return to the heat on high until it hits a boil. Then lower the heat to a low-medium simmer until the rice is cooked and hits a soft consistency you like. It will take about 45-60 minutes depending on your stove. Here's the consistency after boiling, but before adjusting with more water.
Once the rice is cooked to the doneness you like, which should be very soft way beyond al dente and beyond how you'd eat it outside of soup, adjust the water and seasoning levels. When I measured this it was all of the reserved water, and more salt, but I pulled back on both of these so you have room to adjust at the final stage to your taste. You don't want the soup like a thick paste, but you don't want it so thin that you only see water when looking down at the pot.
Remove from the heat, serve with garnish and accoutrement, and enjoy!
*For the rice scoop, I used the scooper included with my rice cooker. For the most accurate way to follow this recipe, please weigh your rice in grams.