Use tomato paste - If a soup has a tomato based broth adding tomato paste is an easy way to thicken the broth.  Puree a vegetable - You can also use one of the vegetables in the stew to thicken the broth. Cauliflower or squash works very well for this because they puree easily. Just mash them after cooking to use as a thickening agent. Add cornstarch or flour - These traditional thickening agents work best when they are added after pressure cooking the meat and vegetables. Mix either with cold water and stir it into the stew. Then use the saute function of Instant Pot to cook for a few minutes until it thickens.