Heat oil in a pressure cooker. Add halved onions and ginger, cut side down. Allow the onion and ginger to char without moving them. Add star anise to roast a bit.
Add cilantro, cloves, coriander, and chicken to the pot, then cover with 2 quarts of water, fish sauce, and the sugar. Seal the pressure cooker and bring it to high pressure over high heat. Cook for 20 minutes, then shock pot under cold running water in the sink to rapidly release the pressure.
Open pressure cooker. Transfer chicken to a plate and remove flesh from bones. Pour broth through a fine mesh strainer into a clean pot and discard solids. Skim any scum off the surface of the broth, leaving any small bubbles of fat intact. Season broth with more fish sauce and sugar, as needed and to suit your individual taste.
To serve, place cooked pho noodles into bowls. Top with chicken meat, sliced onions, and cilantro. Pour hot broth over chicken and noodles. Serve immediately.