Heat oven to 375°F. Cook pasta as directed on package, using minimum cook time; drain. In 12-inch nonstick skillet, cook bacon over medium-high heat until crisp. Reserving drippings in skillet, remove bacon to drain on paper towels. Crumble bacon; set aside.
Add onion and thyme to drippings; cook over medium-high heat, stirring occasionally, until onion is golden brown. Stir in flour; cook 1 minute, stirring constantly. Stir in broth and milk; heat to boiling. Boil, stirring constantly, until thickened. Stir in carrots, tuna, corn, pasta and mustard; heat to boiling. Fold in bacon. Spoon into ungreased 2-quart casserole.
In food processor, place bread. Cover; process with on-and-off pulses several times to make bread crumbs. In small bowl, mix bread crumbs and butter. Sprinkle over tuna mixture.
Bake uncovered 20 to 25 minutes or until mixture is bubbly and topping is golden brown.