To make broth, add chicken, ginger, onion, salt and mushroom seasoning (if using) to a large pot. Fill the pot with water, enough to cover ingredients.
Boil pot on high heat. Reduce to a simmer. Let the stock simmer uncovered for 40 minutes, or until chicken is done.
Use a ladle to skim any scum and foam off the top of the broth.
Once chicken is cooked, remove from pot. Set aside to cool.
Add rice to pot of broth. Cover and simmer for 20 minutes or until rice expands and is fully cooked. Stir frequently to keep rice from sticking together. While rice is cooking, shred chicken and prep garnishes.
Once porridge finishes cooking, check consistency. If you want it thicker, continue cooking. If you want it looser, add water or broth. Adjust seasoning to your preferred taste using fish sauce, additional mushroom seasoning and/or salt.
Ladle "cháo" (rice porridge) into bowls, top with "gá" (chicken). Garnish with black pepper, green onion, cilantro and a squeeze of lime.