Potato and Celery Root Gratin with Gruyère

INGREDIENTS

• Unsalted butter for baking dish
• 1 Tbs. olive oil
• 1/2 cup finely chopped shallots
• 1 tsp. chopped fresh thyme
• 2 cups heavy cream
• 1 tsp. Dijon mustard
• 2 tsp. kosher salt
• 1/4 tsp. freshly ground pepper
• 1 1/2 lb. russet potatoes, peeled and sliced 1/16 inch thick
• 1 1/2 lb. celery root, peeled and sliced 1/16 inch thick
• 4 oz. Gruyère cheese, shredded
• 1 oz. Parmigiano-Reggiano cheese, grated
• 2 tsp. chopped fresh chives or flat-leaf parsley (optional)