Celery Root and Potato Puree with Roasted Jerusalem Artichoke Croutons

Ingredients

• 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)