I generally use butternut squash instead of pumpkin as most of the varieties sold in supermarkets here aren't the best for eating. If I'm using up old lanterns then I roast first, puree, and then put it in a sieve over a bowl to get rid of some more water.
The secret for pumpkin soup is not putting too much water. You just cover the pumpkin but it won't float. I usually put a whole brown onion, 3 or 4 carrots and then the pumpkin. Add water only to the vegetables line, cook for 45-1h covered, in the last 15 add a branch of fresh rosemary , then remove rosemary and put in the blender until creamy. I have it like that, but you can put back in the pot and add some cream.
I soften an onion and/or leek in a knob of butter. Chuck in chopped up pumpkin peeled or unpeeled along with a couple of peeled chopped sweet potatoes. Add a small amount of veg or chicken stock and pepper. Simmer until tender. Add some fresh or dried sage and blend. Stir through a generous blob of creme fraiche and gently heat until nice and hot. Enjoy with a hunk of naice bread.