In a large pot, combine the diced potatoes, turnips, chopped leeks, and fresh thyme sprigs with 3/4 liter of water.

Season with salt, pepper, and a pinch of nutmeg.

Bring the mixture to a boil over medium-high heat, then reduce the heat to low and let it simmer for about 20 minutes or until the vegetables are tender.

Remove the thyme sprigs from the pot.

Using a blender or immersion blender, puree the soup until smooth.

Alternatively, pass the soup through a sieve to achieve a silky texture.

Return the soup to the pot over low heat.

Add the butter and stir until melted and incorporated.

In a separate saucepan, heat the milk until warm but not boiling.

Gradually pour the warm milk into the soup, stirring continuously until well combined and creamy.

Season the soup with additional salt and pepper if needed.

Serve the soup hot, garnished with chopped fresh chervil.