Heat oil in a pressure cooker over high heat until smoking. Add onions and ginger, cut side down. Cook without moving, reducing heat if smoking excessively, until onion and ginger are well charred, about 5 minutes.
Add cilantro stems (reserving leaves for serving), star anise, cinnamon, cloves, fennel seed, coriander, and chicken feet, legs and pork belly to the pot. Add 2 quarts of water, the fish sauce, and the sugar to the pot. Seal the pressure cooker and bring it to high pressure over high heat. Cook on high pressure for 20 minutes, then shock under cold running water in the sink (or release pressure valve if using an electric pressure cooker).
Open pressure cooker. Remove chicken legs and pork belly. Transfer chicken legs to a plate. Place pork belly into a zip seal bag and immerse in an ice bath to chill for slicing. Pour broth through a fine mesh strainer into a clean pot and discard solids. Skim any scum off the surface of the broth using a ladle, but leave the small bubbles of fat intact. Season broth to taste with more fish sauce and sugar if desired.
To serve, place re-hydrated pho noodles in individual noodle bowls. Slice pork belly. Top with chicken legs, pork belly, sliced onions, and scallions. Pour hot broth over chicken and noodles. Serve immediately, allowing guests to add herbs, bean sprouts, chilis, lime, and sauces as they wish.