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Peel the potatoes, rinse them and chop them in chunks. Peel the parsnips, remove the ends and also chop them into chunks. Add the potato and parsnip chunks to a large pan, fill with water and some salt and bring to a boil.
The exact time depends on the thickness of the chunks. Meanwhile remove the stems of the thyme. Heat butter along with milk and thyme leaves until lukewarm and set aside, this way the thyme flavour will enhance.
Don't use too much thyme, you just want a hint of thyme.
Drain the potatoes and parsnips as soon as they're soft. You can rub them through a fine sieve with a large spoon for the smoothest puree. You can also use a potato masher, it will be slightly less smooth.
Gradually add the milk mixture and season with pepper and salt. Theres no need to sift the milk first, because they're not very many thyme leaves in relation to the puree. Enjoy!