Put parsnips in pot, season with salt and cover with water. Place over medium heat and bring to a simmer. Cook until tender - the tip of a paring knife should easily go through without resistance, approximately 15 minutes.
In a medium saucepan place the cream, thyme sprigs and garlic cloves over low heat and bring to a simmer.
Drain parsnips and reserve cooking liquid. Place parsnips in a food processor with butter, or extra-virgin olive oil and a couple of tablespoons of reserved cooking liquid. Begin to process and add strained heavy cream mixture. Season with salt and pepper, to taste, and puree until very smooth.