Prepare pho noodles. Place in large bowl and cover with boiling water for 30 to 45 minutes, stirring occasionally to loosen up. Set aside.
Heat oil in insert in Instant Pot on sauté setting until hot. Add halved onions and ginger, cut side down, and cook without moving, until well charred, about 5 minutes. Add spices (cloves, coriander, star anise, fennel, cinnamon stick and cardamom) and cook 1 more minute until fragrant. Turn off sauté function.
Add water to the pot and then carefully add the chicken, diced apple, chopped cilantro, fish sauce, rock sugar and salt. Seal the pressure cooker and cook at high pressure for 15 minutes.
When done, turn off Instant Pot and let pressure naturally release for 10 minutes, release any remaining pressure manually and carefully open lid. Remove chicken thighs to plate and cool slightly.
Pour broth into cheesecloth-lined strainer (or fine mesh strainer) into a clean pot, discard solids. Skim any scum and some fat, if necessary, leaving some surface fat on the broth. Bring to a steady simmer on the stove. Taste for seasonings, adding more salt, pepper and/or fish sauce, if needed.
Remove skin from chicken and meat from bones, shred. Prepare soup bowls; put a serving of strained noodles in bottom of each bowl, top with some chicken, then ladle hot broth over top. Serve immediately with condiments, letting each person choose toppings as they wish. Happy slurping!