Place carrots in a Dutch oven and cover with water; bring to a boil. Reduce heat; cover and simmer until tender, about 10 minutes. Drain and set aside.
In a large saucepan, saute onions in 4 tablespoons butter until tender. Stir in the flour, salt, celery salt, mustard and pepper until blended. Gradually add milk. Bring to a boil; cook and stir for 2 minutes. Stir in cheese until melted. Add carrots and stir to coat.
Transfer to a greased 13-in. x 9-in. baking dish. Melt remaining butter; toss with bread cubes. Sprinkle over the carrots. Bake, uncovered, at 350° for 40-50 minutes or until heated through.