Sauté onion in olive oil until softened, about 3 minutes.
Add garlic and ginger and sauté for 1 minute.
Add pumpkin puree, vegetable broth, nutmeg, and turmeric. Stir well.
Close the lid and set the Instant Pot to high pressure for 5 minutes.
Allow for a 10-minute natural pressure release, then carefully release any remaining pressure.
Using an immersion blender, blend the soup until smooth and creamy.
Stir in heavy cream.
If desired, whisk cornstarch mixture and slowly add to the soup while the Instant Pot is on sauté mode, stirring until thickened.
Season with salt and pepper to taste.