If making broth ahead of time, remove and discard the fat layer that solidifies on top. Strain the broth, reserving the bones and optional secret ingredient for return. Discard all other solids.
Bring the clear stock to a simmer and season with fish sauce, a touch of rock sugar (if needed), or seasoning powder. The flavor should be slightly bold, as it will balance out with noodles and toppings later.
Place the spice bag into the simmering broth and let it steep for up to 1 hour, or until the classic Pho aroma becomes noticeable. Then remove the spice bag to avoid overpowering the broth.
If using tendon, brisket, flank, or other cooked meats, prime them by soaking and blanching. Add them to the simmering broth and cook until tender—brisket and flank about 1 hour, tendon around 90 minutes. After cooking, plunge the meats into an ice-cold water bath to stop cooking and lock in juiciness.
Slice the reserved rib meat and organize all toppings in airtight containers for easy assembly. If using tripe, soak it in vodka for 5 minutes, blanch briefly, chill in ice water, then thinly slice. Thinly cut raw eye of round for rare topping, and prep meatballs as desired.
Wash and prep all herbs and vegetables. Slice chili peppers, quarter limes, and set everything on serving platters. Thinly slice the green part of the scallions for garnish and add the white parts into the broth. Shave the white onion into ultra-thin slices for bowl topping.