Add turnips, milk, thyme and garlic to a medium-sized sauce pot. Set over medium heat and place the lid over the pot so it is just covering. Bring to a gentle simmer and cook for 20 to 30 minutes until tender - the tip of a paring knife should go through without resistance.

Remove the thyme and remove 2 cups of the liquid. Drain and place turnips back into the same pan and blend using an immersion blender or place to a food processor. Add butter and about 1 cup of the milk cooking liquid and blend. Season with salt and pepper and puree until smooth. Add more of the milk if necessary.