I usually saute carrots with the onions and garlic but roasting does sound like it would give added depth. I'm making soup tomorrow and will give it a try.
@ellesu
I added about an ounce of real butter and sauted the onions and I found a spare satsuma, so I've thrown that in. I'll remove it before blending. The only other ingredient is 1½pints of hot chicken stock (I used buillon powder and vege is just as good). Simmer for 20 minutes, blend and serve.