2 medium celery roots, trimmed, peeled, cut into 1-inch cubes (about 8 cups)
1 pound potatoes, peeled, cut into 3/4-inch cubes (about 3 cups)
2 cups low-salt chicken broth
2 cups whole milk
3 garlic cloves, peeled
3 fresh thyme sprigs
1 fresh bay leaf
4 tablespoons unsalted butter, divided
8 ounces Jerusalem artichokes (also called sunchokes), scrubbed
1/2 tablespoon extra-virgin olive oil
1 teaspoon chopped fresh thyme (for garnish)
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 3 1/2 tablespoons butter. Season with salt and pepper.
Preheat oven to 425 F. Cut Jerusalem artichokes into 1/2" cubes. Place in medium bowl; add oil, sprinkle with salt and pepper;
toss to coat. Dot with remaining 1/2 tablespoon butter. Transfer to rimmed baking sheet; roast until tender and golden
brown, turning occasionally, about 25 minutes.