Celery Root and Potato Puree with Roasted Jerusalem Artichoke Croutons

Name: Celery Root and Potato Puree with Roasted Jerusalem Artichoke Croutons
Ingredients: 2 medium celery roots (celeriac; 1 3/4 pounds total), trimmed, peeled, cut into 1-inch cubes (about 8 cups); 1 pound russet 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 (1/2 stick) 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); Ingredient info: Jerusalem artichokes, the tubers of a variety of sunflower, are available in the produce section of some supermarkets and at farmers' markets.