Cook carrots in lightly salted water until barely tender, about 10 minutes; drain. In a 3-quart saucepan, melt butter and saute onions, about 3 minutes. Stir in flour, salt and mustard; cook 1 minute. Slowly stir in milk. Cook, stirring constantly, until smooth and thickened. Stir in white pepper and celery seed.
In a 2-quart casserole dish, arrange one-third of carrots, pour one-third of sauce over carrots, place one-third of cheddar cheese over sauce. Repeat until all the carrots, sauce and cheddar cheese are used. Sprinkle casserole with KRAFT Parmesan cheese. Top casserole with the buttered bread crumbs and additional cheddar cheese, if desired.
Bake at 350 degrees F for 35 to 45 minutes or until bubbly and golden brown.