I buy he mix veg stew pack from Tescos.... It contains - sweed, carrots, parsnips, onions. I also add some extra onions (as the ones in the bag are realy small) and some garlic.
1)Fry of the onions and garlic with the oil of your choice (I like real olive oil).
2)Add the roughly chopped and peeled veg.
3)Add a kettle of boiled water (some stock if you like)
4)Simmer until the veg is soft.
5)Let cool then put in food processor to make a smooth soup.
I sometime have to add more water to make it thinner.... But you can vary that to your own preference.
Plus you might need to add salt and pepper. I never bother, but then again I never season anything!!!
I get about 4/5 servings out of one £1ish bag of stew veg!