Press the Sauté button twice on the Instant Pot to set the temperature to High. Once the Instant Pot display reads HOT, pour in 1 tablespoon oil. Add 1 large yellow onion, 2 scallions, 1 head garlic, and 5 inches fresh ginger root to the pot, cut side down, without overlapping each other. Let the aromatics char for five minutes—don’t move them at all during this time.
Meanwhile, if your chicken pieces come with skin on, remove this skin to prevent the pho broth from becoming too greasy.
Pour 2 quarts water into the Instant Pot. Add 1 ½ pounds bone-in chicken legs or chicken thighs, 1 small bunch cilantro, 1 teaspoon fennel seeds, 1 teaspoon coriander seeds, 4 whole cloves, 3 whole star anise, 1 to 2 inches cinnamon stick, ¼ cup fish sauce, 1 tablespoon chicken bouillon powder, 1 tablespoon sugar, and ½ teaspoon salt.
Close the Instant Pot lid. Press the Pressure Cook button (or Manual if you have an older model) and set the mode to High Pressure. Set it to pressure cook for 10 minutes.
Meanwhile, boil 8 ounces dry rice noodles according to package directions (usually around 7 to 10 minutes). Drain and submerge in cold water while you wait for the broth to finish cooking.
Once the Instant Pot turns off, allow the pressure to naturally release for 20 minutes. Then release any remaining pressure. Carefully remove the chicken legs and discard the bones.
Next, pour the pho broth through a strainer into a large bowl. Taste for seasoning and more salt, if needed.
Portion the cooked rice noodles and deboned chicken meat into bowls. Ladle strained pho broth over each bowl. Serve with toppings of your choice.