In a small pan with medium heat, toast the coriander, green cardamom pods, star anise pods, cloves, and cinnamon stick until they become fragrant and a little burnt. Set them aside and let the spices cool.
With an open flame, char the two onion halves and piece of ginger until the outsides blacken. This will take about 10 minutes.
Meanwhile, set your instant pot to saute and add in the oil.
Once the oil gets hot, brown the chicken legs. You are not looking to cook the chicken just to get the skin browned and a little crispy. Approx. 10 minutes total.
Once the chicken is ready, put all the spices in a spice bag and throw it into the instant pot along with the charred onions, ginger, and fill the instant pot to the top mark on the inside of the pot. For me this was approx 10 cups of water. It may vary for you.
Set the instant pot to high pressure for 1 hour.
Once the time is up, release the pressure, carefully take the lid off, and add in the fish sauce. Taste it and then add in as much or as little salt as you'd like. I normally do about 1 teaspoon.
At this point you can either jar up the broth in clean glass jars and save it for another day or you can serve it immediately. I like to eat it with slices of chicken breast, pho noodles, bean sprouts, some thai basil, and a wedge of lime. But honestly the options are endless!