Blanch the bones. This step is SO IMPORTANT. If you want a clear, beautiful pho broth, blanch your bones. Do this by dividing the bones between two large stockpots and cover with cold water. Bring to a boil over high heat and simmer for 15-20 minutes before draining and rinsing the bones with water.
Roast the bones and the vegetables. Preheat the oven to 450 degrees F. Transfer the bones, onion, garlic, and ginger to the roasting pans. Don’t pile them all on top of each other- use two roasting pans. Roast for 30 minutes before gently tossing the bones, and roasting for an additional 15-30 minutes more. In other words, roast for at least 45-60 minutes.
Toast the spices. As the bones are roasting, add all of your spices (the star anise, cinnamon, cloves, cardamom pods, coriander seeds, and fennel seeds) to a large, dry skillet over low heat. Not medium heat. LOW HEAT. Toast your spices, stirring often to prevent burning, until fragrant, approximately 5 minutes. Divide the spices in half and transfer to your cheesecloth. Make two spice sachets by gathering at the top and tying with kitchen twine.
Transfer the roasted bones back to the stockpots. But not before washing the stockpots first. Make sure you wash your pots after the bones were blanched and drained. Transfer the bones back to the stock pots and scrape up any remaining bits and juices remaining in the roasting pan using a metal spatula and a little water, if needed. Add to the pot with the bones (don’t worry, all those brown bits are FLAVOR!).
Bring to a boil. With the bones, spice sachets(one per pot), onion, garlic, and ginger divided between the two pots, fill each pot with approximately 12 cups water (or until bones are fully submerged), 1/3 cup fish sauce and 2 tablespoons sugar. Bring to a boil.
Simmer the bones. Reduce heat to low and simmer, with the lid slightly ajar, skimming any foam or excess fat, occasionally (if you blanched your bones, you shouldn’t see much foam). Simmer for at least 6-12 hours, ideally 24 hours (do not leave the stove running overnight. Simply cool and store in the refrigerator and continue to simmer the next day). Add more water if needed to make sure the bones stay submerged.
Strain the broth. Once the bones have simmered and your broth is ready, you will need to strain the broth through a fine mesh strainer essentially separating the pretty, clear broth from the bones and spices. Set aside the broth to cool and reserve the bones.
Don’t forget about the meat. Whether you eat the meat still left on the bones in a bowl of soup or in sandwiches, I can almost guarantee that there is a TON of delicious meat waiting to be picked from the bones. Don’t let it go to waste! Discard the meat-free bones, herbs, and other bits that were used to make the broth.
Skim the fat from your broth (optional). Add a couple handfuls of ice to your broth to expedite cooling and cover your pot with a lid. Transfer the broth to the refrigerator and allow broth to cool fully. The result will be a hard, thick layer of fat on the top and a bottom layer that is your pho broth (which should look like gelatinous brown jello). If desired use a fork to scoop off the top layer of fat.