Somewhat akin to elcee's recipe, here's another using your ingredients with the exception of the sweet potatos:
Ingredients:
2 tbsp olive oil
1 onion, diced
4 cloves garlic, minced
4 carrots, sliced
3 stalks celery, diced
2 small or one large butternut squash, peeled and chopped
5 cups vegetable broth (we prefer chicken broth)
2 tbsp chopped fresh sage (If you have to use dried, increase to 4 tablespoons)
1/2 cup soy milk
salt and pepper to taste
Preparation:
In a large soup pot, sautee the onion and garlic in olive oil until onions turn soft, about 3 to 5 minutes.
Add the carrots and celery and cook for another 3 to 5 miutes.
Add the squash and stir just to coat, then add the vegetable broth and sage. Bring to a boil, then reduce heat to a slow simmer. Allow to cook for at least 25 minutes, or until squash is soft.
Using a potato masher or a large fork, mash the squash until smooth, or, alternatively, you can puree the soup in a food processor or blender.
Stir in the soy milk and season with salt and pepper to taste...
I would, then, bake the sweet potatos with the skins on at about 350 degrees F (sorry, we're in the U.S.) for 45 minutes or until the skins start to turn brown. Split in half and drizzle with butter and a little real maple syrup...