Cook carrots in boiling water 7 to 9 minutes or until just crisp-tender; drain.
Meanwhile, in a large saucepan, cook onion in the 1/4 cup butter or margarine until tender. Stir in flour, salt, mustard, celery salt, and pepper.
Add milk all at once. Cook and stir until bubbly. Stir in cheese spread until melted. Add carrots and stir to coat. Transfer to12x7-1/2x2-inch (2-quart rectangular) baking dish.
Toss bread cubes and the 1/3 cup melted butter or margarine to coat; sprinkle over carrots.
Bake, uncovered, in a 350 degree F oven about 30 minutes or until the mixture is bubbly, and the bread cubes are lightly toasted. Makes 12 to 16 side-dish servings.