Press the "Saute" button on the Instant Pot, set the time to 10 minutes and add butter. When the butter melts, add the onion and cook, stirring occasionally, for 5 minutes or until golden-brown. Press the "Cancel" button to stop sauteing.
Add the chicken broth (or vegetable broth) and pumpkin puree to the Instant Pot and mix until combined (and the onions are mixed with liquid). Add brown sugar, cinnamon, ground ginger, salt, pepper and Italian seasoning and mix until combined. DO NOT add heavy cream at this point.
Close the Instant Pot lid and turn the valve to a Sealing position. Press the "Pressure Cook" or "Manual" button and set the time to 5 minutes on High pressure.
When it's done cooking, let the pressure release naturally for 5 minutes, then quick release (QR) the remaining steam.
Add the heavy cream to the Instant Pot. Use the immersion blender to blend the pumpkin soup. Add salt and pepper to taste if needed.