Finely dice onion and celery. Chop the pumpkins and sweet potato into small cubes.
Place the butter in a large pot, along with the onion and celery. Then saute until tender.
Place the pumpkins and sweet potato in the pot, cover with water, then bring to a boil.
Once boiling turn down the heat to medium, add chicken stock and stir through. Then continue to cook until the pumpkin is starting to fall apart and the sweet potato is really tender.
Turn off the heat, and blend with a hand blender (refer to notes for other options)
Add an extra cup of water if it is too thick for you.
Serve in a bowl with cream or sour cream.
Notes
If you do not have a hand blender, you can use a masher combined with a hand mixer or even a blender (doing a bit at a time)