Toss the leeks and carrots in the pressure cooker or Instant Pot (make sure it’s at least 6-quarts), and toss in your bones (frozen is fine). Then, cover with water (make sure you don’t fill more than two-thirds capacity!).
Add the vinegar (optional) fish sauce, and dried shiitake mushrooms (optional).
Then, cover and lock the lid, and program your Instant Pot to cook under high pressure for 30 to 50 minutes. (I let it go for 50 minutes if I’m cooking meaty shanks or oxtails). If I’m not in a rush, I set my Instant Pot on high pressure for 2 hours—the broth only gets better with more time.
If you're using a stovetop pressure cooker, place the pot on a burner set on high heat. Once the indicator pops up showing that the contents of the pot have reached high pressure, immediately decrease the temperature to the lowest possible setting to maintain high pressure (low is normally adequate). Set the timer for 30 minutes. When the timer goes off, turn off the burner and remove the pot from the heat.
When the broth is done cooking in the Instant Pot or stovetop pressure cooker, let the pressure release naturally (10-15 minutes).
Remove the lid, skim of the scum (if you desire), and strain the broth. Taste for seasoning and adjust if needed. You can keep it on the blander side if you will be cooking with it!