Preheat the oven to 425 F.
Line a baking pan with parchment paper, and roast the chicken, flanken, and onion for 30 minutes.
Remove the meat and onion from the oven and place it in your Instant Pot.
Chop the carrots and celery into fours and add to the pot.
Toss in the peppercorns and salt.
Add enough water to meet the ‘max’ line: in my Instant Pot that’s 16 cups.
Close the Instant Pot and set it to maximum pressure for 2 hours.
Manually release the pressure and allow it to depressurize.
Remove the chicken, flanken, and vegetables with a mesh ladle.
Place a large strainer lined with cheesecloth over a large pot.
Carefully pour the broth into the strainer and allow it to filter into the pot.
Once the broth has cooled somewhat, place it in the fridge overnight.
The next day, skim the hardened fat off the soup and discard.