Select Sauté on a 6-quart pressure cooker. Heat oil, add onion and cook 4 to 5 minutes until softened. Add apple, garlic and sage; cook 1 minute.
Pour in 1 cup broth and scrape the bottom thoroughly until no browned bits remain.
Add squash, remaining 2 cups broth, salt, pepper and nutmeg. Do not fill above the appliance's maximum line.
Lock the lid, set the valve as directed and cook on HIGH pressure for 8 minutes. The pot will take additional time to come to pressure.
Let pressure release naturally for 10 minutes, then vent the remaining pressure in short controlled bursts according to the manual. Keep hands and face away from steam.
Open the lid away from you. Blend until smooth, then stir in optional cream, vinegar and enough extra broth for the desired consistency.