Root Vegetable Gratin

Name: Root Vegetable Gratin
Ingredients: FOR THE VEGETABLES:; 1 whole Large Celery Root; 6 whole Fingerling Potatoes; 2 whole Turnips; 1 whole Large Golden Beet; 1 whole Large Rutabaga; 1 whole Medium Yam; Fresh Thyme And Kosher Salt, To Taste; 6 dashes Truffle Oil; _____; FOR THE MORNAY SAUCE:; 3-½ cups Heavy Cream, Divided; 3 Tablespoons Butter; 3 Tablespoons Flour; ¼ cups Comte Cheese Shredded; ¼ cups Tellagio Cheese, In Chunks; ½ teaspoons Nutmeg; Kosher Salt And Black Pepper To Taste