Dice the onion.
Melt butter in a large pan and add the diced onion, cook over a medium heat for 3-4 minutes then remove from heat.
Peel and chop the swede, carrots and parsnips into small even chunks, place in the pan with the onion.
Pour over the vegetable stock, make sure the vegetables are completely covered, you can add more stock if necessary.
Add the herbs, salt and pepper, stir and place the pan back on the heat, bring to the boil then reduce heat, cover and simmer gently for 1 hour.
Remove from heat. Using a hand held blender, blend the soup until smooth. Take care to keep the blender head below the surface so it doesn't spray as the soup will be very hot.
If you like your soup with a bit more kick, try adding a teaspoon of chilli powder to the mix before cooking. Great served with warm crusty bread.
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