Combine the pumpkin, onion, potatoes, vegetable broth, salt, pepper, cinnamon, nutmeg, ginger and cayenne pepper in a 6-quart slow cooker. Stir to combine.
Place the lid on the slow cooker and cook for 4 hours on HIGH or until the potatoes and pumpkin are cooked through.
Use an immersion blender to puree all ingredients into a thick soup. Stir in the cream and serve.
Top with pumpkin seeds (pepitas), chopped parsley and/or a drizzle of extra cream (all optional).