In a medium sauce pan melt butter. Add flour and whisk for 1 minute. Add flour, onion, salt, dry mustard, pepper and celery salt. Mix well. Slowly add milk, stirring constantly. Once all milk is added, cook until smooth and thickened.

In a casserole, layer 1/3 of the cooked carrots, ½ of the cheese, 1/3 of the sauce, then repeat, 1/3 carrots, remaining cheese, 1/3 sauce, remaining carrots and remaining sauce. Sprinkle on breadcrumbs.

Bake at 350 for 25 minutes or until bubbly.