Toast the Spices. In a dry skillet over medium-low heat, toast the spices for 2–3 minutes until fragrant. Add coriander and fennel seeds during the last minute to prevent burning.
Prepare the Spice Bag. Place the toasted spices into a cheesecloth, muslin cloth, or spice bag. Tie it securely to keep everything contained.
Simmer the Broth. Add the spice bag to your simmering pho broth during the last 30-40 minutes of cooking. Remove once the broth is infused to your liking.
