Peel the parsnips and chop roughly into even sized pieces.
Slice the onion.
Heat the oil in a oven proof pot and saute the vegetables for 4 to 5 mins until they begin to soften but not to colour.
Add thyme (chopped), enough water to cover the vegetables and the stock cube and bring to a simmer.
Put on the lid and pop into the oven (bottom oven of the AGA) for 30 mins or until the vegetables are cooked through.
Or continue to cook on the stove top at a simmer until the vegetables are cooked through. If cooking on the stove top check from time to time that the vegetables have enough liquid, you dont want them to burn!
When the vegetables are cooked let them cool and then blitz the contents of the pot to a puree.
Return the puree to the pan and add enough milk and water to make the consistancy that you like for your soup.