I just made a pretty killer pumpkin/sweet potato soup last month. This is how I did it.
Wrap a couple of sweet potatoes in foil and put them in the oven at 350 for two hours
Cut your pumpkin in half, de-seed, and rub some olive oil over the pumpkin, so it doesn't burn on the pan and let it cook next to the sweet potato. You could also salt and season the seeds and let them roast in the oven too, just add some oil and check them. It would be a great garnish for when the soup is done.
Chop up an onion and a clove of garlic
Gather your spices - Nutmeg, Clove, Cinnamon, Salt, Ginger.
When everything is cooked, scrape out the pumpkin, and peel the potatoes, set them aside in a bowl together.
Cook the onions and garlic until translucent, add some spices to flavor the onions a bit.
Add in some veggie broth and bring your bowl of pumpkin + yam over and stir it around a bit.
Once it comes to a boil, let it simmer and stew at a lower temperature.
You have a few options at this point.. if you're wanting a "creamy" bisque style soup I would recommend soaking cashews before anything to make a "cashew cream" just add your soaked cashews covered in water to a blender with a dash of nutmeg. You can add this "cream" at the end to thicken and make the soup a little richer. If this is too much work, than you can just add a bit of salted soymilk, which should be fine.
Last step would be either blend with an immersion blender, or just transfer the soup to a regular blender and blend in batches. You should come out with a super nice creamy pumpkin soup. Just adjust the spices to taste. heart