1. Heat 1 3/4 cups milk, onion, and bay leaf in a large saucepan to 180° or until tiny bubbles form around edges (do not boil). Cover and remove from heat; let stand 15 minutes.
2. Cook pasta according to package directions; drain.
3. Strain milk mixture through a colander over a bowl; discard solids. Return milk to saucepan over medium heat. Combine remaining 1/2 cup milk and flour in a small bowl, stirring with a whisk until well blended. Gradually stir flour mixture and salt into warm milk, stirring constantly with a whisk. Bring mixture to a boil, stirring frequently; cook 1 minute, stirring constantly. Remove from heat; let stand 6 minutes or until mixture cools to 155°. Gradually add fontina and Comté cheeses, stirring until cheeses melt. Stir in pasta and truffle oil. Spoon mixture into a 2-quart broiler-safe glass or ceramic baking dish.
4. Preheat broiler.
5. Place bread, Parmesan cheese, and garlic in a food processor; process until coarse crumbs form. Drizzle with olive oil; pulse until fine crumbs form. Sprinkle breadcrumb mixture over pasta. Place dish on middle rack in oven; broil 2 minutes or until golden brown.