parsnip soup is fab, so is carrot. Turnip isn't, though you can use swede if making a general mixed winter veg soup. My quantities are vague guesswork, I do it by eye.
Clear white stock is useful if you have it. Start by sweating 1 or 2 rough-chopped onions, and garlic if liked, a tiny bit of finely chopped celery if you have it, in oil or fat, but make transparent, do not brown, then add about 500g of main vegs and 500 to 1000ml stock (or water if necc) enough to cover well. V. slow simmer until softer than if you were serving vegs with a meal, let it off the boil, whiz up, correct the seasoning, add half a glass of white wine if you have it. For economy you can bulk up with potatoes esp if making a very big one. Use white pepper and be sparing with the salt. If you have left-over turkey fat or similar, use that instead of oil, it adds flavour. Or sweat bits of fatty bacon or left-over fatty meat, to do the onions in. Don't try to cook with lamb fat though, it tastes and smells horrible. You can finely chop a bit of parsley to spinkle on the bowls if you have it, or add a swirl of cream (sour or otherwise) if you like. Do your sweating in the pan you will later simmer in, to avoid wasting the tasty coating on the pan.
Some people use stock cubes, but I can taste them. Knorr do good bottled stock concentrates. After whizzing, it may be too thick. If you add more boiling water, it will need re-seasoning as the flavour will be diluted. You can liven it with a little Worcester sauce or tabasco, but so little that it is not identifiable. Do not add tomato or its derivatives to this one.