-
Combine first 7 ingredients in heavy large pot. Add enough water to cover. Sprinkle with salt.
-
Bring to boil, reduce heat to medium, and simmer with lid slightly ajar until vegetables are tender, 15 to 20 minutes.
-
Drain; return to pot.
-
Discard thyme sprigs and bay leaf. Stir over medium heat to dry vegetables.
-
Using potato masher, mash vegetables until coarsely pureed. Mash in butter. Season with salt and pepper.