Cook carrots in boiling salted water to cover in a large saucepan 5 minutes or until tender. Reserve 3 tablespoons liquid, and drain. Spoon carrots into a 2-quart baking dish. Set aside.
Combine mayonnaise, reserved carrot liquid, onion, and next 4 ingredients. Spoon over carrots.
Combine breadcrumbs and butter; sprinkle over casserole.
Bake at 375° for 20 minutes or until thoroughly heated.