Take the full rotisserie chicken and remove the meat from the bones, add the meat into a separate plate. Take all the discard, bones, skin, and juices and transfer to a large Dutch oven or pot. (Be sure that the pot is 5 quarts or larger.)
In a dry skillet, add the pho spices and toast on medium high heat for about 2-3 minutes.
Carefully place the toasted spices into the baggie that came with the spices and tie the bag tight. Add the bag to the pot.
In the same skillet, add the halved onion and cut up ginger to the skillet and toast until slightly charred and toasted. Transfer the charred onion and ginger to the pot.
Fill the pot with water and add the chicken bouillon powder, salt, sugar, and fish sauce.
Bring it to a boil, reduce the heat to medium low and simmer for 45 minutes. Cleaning the foamy discard as it simmers.
At the 45-minute simmer mark carefully remove the bag of pho spices. (Taste the broth and if you want the broth to taste strong, leave the spice bag in longer. It is based on desired taste.)
Continue simmering the pho broth for about 15-25 minutes, no longer than 2 hours. I like to keep it simmering for 1 hour.
Carefully remove and discard everything from the broth. (You can also pour the broth through a fine strainer for a smooth, clear broth.)
Once everything is removed from the broth, adjust the seasoning if need be. (Taste and adjust the seasonings to desired taste preference.)
Turn off the heat and assemble your pho.