Parboil carrots until barely tender, in salted water and drain.
Saute onion in butter in a frying pan for 2 or 3 minutes.
Stir in flour, salt and pepper, mustard and celery salt.
Add milk; cook slowly, stirring until smooth.
In a 2 quart casserole dish place a layer of carrots, then cheese.
Pour over sauce; top with buttered crumbs.
Bake, uncovered for 45 minutes at 350Â°F.
Can be made the day before.